soc: fsl: qe: remove set but not used variable 'mm_gc'

drivers/soc/fsl/qe/gpio.c: In function qe_pin_request:
drivers/soc/fsl/qe/gpio.c:163:26: warning: variable mm_gc set but not used [-Wunused-but-set-variable]

commit 1e714e54 ("powerpc: qe_lib-gpio: use gpiochip data pointer")
left behind this unused variable.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>