wilc1000: Introduce symbolic names for SPI protocol register

The WILC1000 protocol control register has bits for enabling the CRCs
(CRC7 for commands and CRC16 for data) and to set the data packet
size.  Define symbolic names for those so the code is more easily
understood.

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210227172818.1711071-2-davidm@egauge.net