tty/serial/8250: make UART_MCR register access consistent

Introduce serial8250_out_MCR() and serial8250_in_MCR() routines, that
replace following calls:

serial_out(port, UART_MCR, val)
serial_port_out(up, UART_MCR, val)
serial_in(port, UART_MCR)

This patch is needed in order to integrate reading/writing of MCR
signals via SERIAL_MCTRL_GPIO infrastructure later.

Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>