rtc: at91sam9: remove useless check

rtc->sclk necessarily points to a valid clocks at this point. Else the
probe would have aborted.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>