libertas: add support for Marvell SD8688 chip

Use RxPD->pkt_ptr to locate eth803 header in the packet
received since SD8688/v10 firmware allows a gap between
RxPD and eth803 header.

Set SDIO block size to 256 for CMD53.
The maximum block size for SD8688 WLAN function is set
to 512 in TPLFE_MAX_BLK_SIZE. But using 512 as block size
results upto 2K bytes data (4 blocks) being transferred
and causes buffer overflow in firmware.

Both changes above are backward compatible with earlier
firmware versions for SD8385/SD8686.

The SDIO_DEVICE_IDs for SD8688 chip are added in
include/linux/mmc/sdio_ids.h
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>