rtc: gamecube: Always reset HW_SRNPROT after read

This register would fail to be reset if reading the RTC bias failed for
whichever reason.  This commit reorganises the code around to
unconditionally write it back to its previous value, unmap it, and
return the result of regmap_read(), which makes it both simpler and more
correct in the error case.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220823130702.1046-1-linkmauve@linkmauve.fr