rtc: rc5t619: Fix an ERR_PTR vs NULL check

The devm_kzalloc() function returns NULL on error, it doesn't return
error pointers so this check doesn't work.

Fixes: 540d1e15 ("rtc: rc5t619: Add Ricoh RC5T619 RTC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200407092852.GI68494@mwanda


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>