wilc1000: Rename SPI driver from "WILC_SPI" to "wilc1000_spi"

The name "wilc1000_spi" follows normal Linux conventions and also is
analogous to the SDIO driver, which uses "wilc1000_sdio".

Signed-off-by: David Mosberger-Tang <davidm@egauge.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211209044411.3482259-2-davidm@egauge.net