wilc1000: Make SPI transfers work at 48MHz

For CMD_SINGLE_READ and CMD_INTERNAL_READ, WILC may insert one or more
zero bytes between the command response and the DATA Start tag (0xf3).
This behavior appears to be undocumented in "ATWILC1000 USER GUIDE"
(https://tinyurl.com/4hhshdts

) but we have observed 1-4 zero bytes
when the SPI bus operates at 48MHz and none when it operates at 1MHz.

This code is derived from the equivalent code of the wilc driver in
the linux-at91 repository.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210227172818.1711071-1-davidm@egauge.net