mips: cm: add missing iounmap() on error in mips_cm_probe()

Add the missing iounmap() of iounmap(mips_gcr_base) before
return from mips_cm_probe() in the error handling case.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>