Polling mode provides better throughput in general by avoiding the
interrupt overhead as the maximum data size one interrupt can handle is
only 512 bytes. So switch to polling mode as the default mode but add
a driver sysfs option wait_mode to allow user manually changing the mode
at run time between interrupt and polling. Also add driver banner
message when the driver is loaded successfully.

When test on a Broadcom BCM47622(ARM A7 dual core) reference board with
WINBOND W25N01GV SPI NAND chip at 100MHz SPI clock using the MTD speed
test suite, it shows about 15% improvement on the write and 30% on
the read:
** Interrupt mode **
  mtd_speedtest: MTD device: 0    count: 16
  mtd_speedtest: MTD device size 134217728, eraseblock size 131072, page
size 2048, count of eraseblocks 1024, pages per eraseblock 64, OOB size
64
  mtd_test: scanning for bad eraseblocks
  mtd_test: scanned 16 eraseblocks, 0 are bad
  mtd_speedtest: testing eraseblock write speed
  mtd_speedtest: eraseblock write speed is 3072 KiB/s
  mtd_speedtest: testing eraseblock read speed
  mtd_speedtest: eraseblock read speed is 6690 KiB/s
  mtd_speedtest: testing page write speed
  mtd_speedtest: page write speed is 3066 KiB/s
  mtd_speedtest: testing page read speed
  mtd_speedtest: page read speed is 6762 KiB/s
  mtd_speedtest: testing 2 page write speed
  mtd_speedtest: 2 page write speed is 3071 KiB/s
  mtd_speedtest: testing 2 page read speed
  mtd_speedtest: 2 page read speed is 6772 KiB/s
** Polling mode **
  mtd_speedtest: MTD device: 0    count: 16
  mtd_speedtest: MTD device size 134217728, eraseblock size 131072, page
size 2048, count of eraseblocks 1024, pages per eraseblock 64, OOB size
64
  mtd_test: scanning for bad eraseblocks
  mtd_test: scanned 16 eraseblocks, 0 are bad
  mtd_speedtest: testing eraseblock write speed
  mtd_speedtest: eraseblock write speed is 3542 KiB/s
  mtd_speedtest: testing eraseblock read speed
  mtd_speedtest: eraseblock read speed is 8825 KiB/s
  mtd_speedtest: testing page write speed
  mtd_speedtest: page write speed is 3563 KiB/s
  mtd_speedtest: testing page read speed
  mtd_speedtest: page read speed is 8787 KiB/s
  mtd_speedtest: testing 2 page write speed
  mtd_speedtest: 2 page write speed is 3572 KiB/s
  mtd_speedtest: testing 2 page read speed
  mtd_speedtest: 2 page read speed is 8806 KiB/s
Signed-off-by: William Zhang <william.zhang@broadcom.com>

Link: https://lore.kernel.org/r/20230207065826.285013-8-william.zhang@broadcom.comSigned-off-by: Mark Brown <broonie@kernel.org>