rtc rx4581: change err type from unsigned char to int in get_datetime()

rtc rx4581: change err type from unsigned char to int in get_datetime()

in response to these warnings posted by dan.carpenter@oracle.com:

New smatch warnings:
drivers/rtc/rtc-rx4581.c:134 rx4581_get_datetime() warn: unsigned 'err' is never less than zero.

Old smatch warnings:
drivers/rtc/rtc-rx4581.c:175 rx4581_get_datetime() warn: unsigned 'err' is never less than zero.

Signed-off-by: Torben Hohn <torbenh@linutronix.de>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>