serial: mctrl_gpio: Fix passing zero to 'ERR_PTR' warning

drivers/tty/serial/serial_mctrl_gpio.c:214
 mctrl_gpio_init() warn: passing zero to 'ERR_PTR'

gpiod_to_irq() never return 0, so remove the useless test
and make code more clear.

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20201031030530.1304-1-yuehaibing@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>