rtc: gamecube: Check the return value of ioremap()

rtc: gamecube: Check the return value of ioremap()

The function ioremap() in gamecube_rtc_read_offset_from_sram() can fail
and return NULL, which is dereferenced without checking, leading to a
NULL pointer dereference.

Add a check for the return value of ioremap() and return -ENOMEM on
failure.

Fixes: 86559400b3ef ("rtc: gamecube: Add a RTC driver for the GameCube, Wii and Wii U")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Link Mauve <kernel@linkmauve.fr>
Link: https://patch.msgid.link/20251126080625.1752-1-vulab@iscas.ac.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>