rtc: sysfs: fix NULL check in rtc_add_groups()

devm_kcalloc() returns NULL, it never returns error pointers.  In the
current code we would return PTR_ERR(NULL) which is success, instead of
returning the -ENOMEM error code.

Fixes: a0a1a1ba ("rtc: sysfs: facilitate attribute add to rtc device")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>