tty: serial: omap: fix Sparse warnings

Fix the following Sparse warnings:

drivers/tty/serial/omap-serial.c:1418:49: warning: incorrect \
	type in argument 2 (different address spaces)
drivers/tty/serial/omap-serial.c:1418:49:    expected void const \
	[noderef] <asn:1>*from
drivers/tty/serial/omap-serial.c:1418:49:    got struct serial_rs485 \
	*<noident>
drivers/tty/serial/omap-serial.c:1426:35: warning: incorrect \
	type in argument 1 (different address spaces)
drivers/tty/serial/omap-serial.c:1426:35:    expected void [noderef] \
	<asn:1>*to
drivers/tty/serial/omap-serial.c:1426:35:    got struct serial_rs485 \
	*<noident>

Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>