rtc: mrst: fix error code in probe()

We should be returning "retval".  The "mrst_rtc.rtc" variable is a valid
pointer.

Fixes: 32b41f93

 ("rtc: mrst: switch to devm functions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>