rtc: gamecube: Add missing iounmap in gamecube_rtc_read_offset_from_sram

The hw_srnprot needs to be unmapped when gamecube_rtc_read_offset_from_sram returns.

Fixs: 86559400 (rtc: gamecube: Add a RTC driver for the GameCube, Wii and Wii U)
Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220511071354.46202-1-yuancan@huawei.com