serial: 8250_fintek: Refactoring read/write method

If we need to access SuperIO registers, It should write register offset
to base_addr and read/write value to base_addr + 1 to perform read/write.
We can make it more simply with write/read functions.

This patch add sio_read_reg()/sio_write_reg()/sio_write_mask_reg() to
reduce SuperIO register operation with lot of outb()/inb().

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>