SERIAL: omap: fix set_mctrl() breakage

c538d20c (and maybe previous commits) broke set_mctrl() by making
it only capable of setting bits in the MCR register.  This prevents
software controlled flow control and modem control line manipulation
via TIOCMSET/TIOCMBIC from working correctly.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>