wilc1000: Check for errors at end of DMA write

After a DMA write to the WILC chip, check for and report any errors.

This is based on code from 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/20210227172818.1711071-3-davidm@egauge.net