serial: 8250: Add MSR/MCR TIOCM conversion wrapper functions

This patch adds wrapper functions to convert MSR <-> TIOCM and also
MCR <-> TIOCM. These functions are used now in serial8250_do_set_mctrl()
and serial8250_do_get_mctrl().

Signed-off-by: Stefan Roese <sr@denx.de>
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Giulio Benetti <giulio.benetti@micronovasrl.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>