serial: max310x: Always use dynamic GPIO ID assignment

Always register GPIOs and use dynamic GPIO ID assignment.
This is no much worth if GPIOs is not used, but helps remove
private driver header and add DT support in the future.
Additionally, patch adds missing uart_unregister_driver()
call if probe() fails.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>