staging: pi433: depends on SPI

The pi433 driver uses SPI interfaces so it should depend on SPI.
Also, the "default n" can be removed since that is already the
default.

Fixes these build errors when SPI is not enabled:

drivers/staging/pi433/pi433_if.o: In function `pi433_probe':
pi433_if.c:(.text+0x1135): undefined reference to `spi_setup'
pi433_if.c:(.text+0x1177): undefined reference to `spi_write_then_read'
drivers/staging/pi433/pi433_if.o: In function `pi433_init':
pi433_if.c:(.init.text+0xb8): undefined reference to `__spi_register_driver'
drivers/staging/pi433/rf69.o: In function `rf69_read_fifo':
rf69.c:(.text+0x102): undefined reference to `spi_sync'
drivers/staging/pi433/rf69.o: In function `rf69_write_fifo':
rf69.c:(.text+0x248): undefined reference to `spi_sync'
drivers/staging/pi433/rf69.o: In function `rf69_read_reg':
rf69.c:(.text+0x290): undefined reference to `spi_write_then_read'
drivers/staging/pi433/rf69.o: In function `rf69_write_reg':
rf69.c:(.text+0x523): undefined reference to `spi_sync'
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/Kconfig

 config PI433
         tristate "Pi433 - a 433MHz radio module for Raspberry Pi"
-        default n
+        depends on SPI
         ---help---
           This option allows you to enable support for the radio module Pi433.