serial: core: Simplify uart_get_rs485_mode()

Simplify uart_get_rs485_mode() by using temporary variable for
the GPIO descriptor. With that, use proper type for the flags
of the GPIO descriptor.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231003142346.3072929-1-andriy.shevchenko@linux.intel.com


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