rtc: sh-rtc: Handle rtc_device_register() failure properly.

Currently if rtc_device_register() fails we have an IS_ERR() on
the wrong pointer, which causes this to always be skipped. Fix
this up to actually check the right pointer. The return value
was always correct, even though the check was wrong.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>