serial: fix serial_txx9.c build warning/typo

Fix kconfig symbol test to use "defined":

drivers/tty/serial/serial_txx9.c: warning: "CONFIG_CONSOLE_POLL" is not defined [-Wundef]

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>