rtc: pcf2123: Add missing error code assignment before test

It is likely that checking the result of 'pcf2123_write_reg' is expected
here.
Also fix a small style issue. The '{' at the beginning of the function
is misplaced.

Fixes: 809b453b ("rtc: pcf2123: clean up writes to the rtc chip")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>