wilc1000: use devm_clk_get_optional()

Use devm_clk_get_optional() for rtc clock: it simplifies a bit
the code.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210806081229.721731-4-claudiu.beznea@microchip.com