tty: serial: max310x: Check return code of gpiochip_remove

The gpiochip_remove function may fail to remove a gpio_chip
if any GPIOs are still requested. This patch informs the caller
of such a senario.

Sparse is warning because the function prototype has a
__must_check annotation.

Sparse warning:
drivers/tty/serial/max310x.c:1223:18: warning:
	ignoring return value of ‘gpiochip_remove’,
	declared with attribute warn_unused_result
	[-Wunused-result]

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>