serial: 8250_mtk: Switch to use platform_get_irq()

platform_get_irq() provides an established error code and error message.
Also, it's better to use dedicated API to retrieve Linux IRQ resource.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/20200618122952.88265-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>