ARM: ep93xx: Don't use modem interface on the second UART

Second UART doesn't have modem interface, so any attempt to use set_mctrl() it
produce unwanted garbage on the line. There's no such 0x100 register offset
for the second UART either.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ryan Mallon <rmallon@gmail.com>