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 <[e-mail]>
Reviewed-by: Link Mauve <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-gamecube.c

···
 	}
 
 	hw_srnprot = ioremap(res.start, resource_size(&res));
+	if (!hw_srnprot) {
+		pr_err("failed to ioremap hw_srnprot\n");
+		return -ENOMEM;
+	}
 	old = ioread32be(hw_srnprot);
 
 	/* TODO: figure out why we use this magic constant.  I obtained it by