serial: 8250_exar: Add ->unregister_gpio() callback

For the sake of reducing layering violation add ->unregister_gpio()
callback and use it in the ->exit() one.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210713095821.7834-1-andriy.shevchenko@linux.intel.com


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