rtc: rtc-sh: Fix up oops in early periodic freq assignment.

With the reordered init order, the rtc device is not registered until
later, while sh_rtc_irq_set_freq() was attempting to assign ->irq_freq
directly, resulting in an oops. This is handled by the upper layers for
us, so just kill off the problematic dereference completely.

Reported-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>