wilc1000: Fix clock name binding

Documentation/devicetree/bindings/net/wireless/microchip,wilc1000.yaml
requires an "rtc" clock name.
drivers/net/wireless/microchip/wilc1000/sdio.c is using "rtc" clock name
as well. Comply with the binding in wilc1000/spi.c too.

Fixes: 854d66df ("staging: wilc1000: look for rtc_clk clock in spi mode")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210428025445.81953-1-tudor.ambarus@microchip.com