rtc: gamecube: Fix an IS_ERR() vs NULL check

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

Fixes: 86559400 ("rtc: gamecube: Add a RTC driver for the GameCube, Wii and Wii U")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220107073340.GF22086@kili