[ARM] 21285 serial: fix build error

drivers/serial/21285.c: In function 'serial21285_set_termios':
drivers/serial/21285.c:280: error: 'tty' undeclared (first use in this function)
drivers/serial/21285.c:280: error: (Each undeclared identifier is reported only once
drivers/serial/21285.c:280: error: for each function it appears in.)

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