mips: ar7: add missing iounmap() on error in ar7_gpio_init

Add the missing iounmap() of gpch->regs before return from
ar7_gpio_init() in the error handling case.

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