serial: fsl_lpuart: remove build warning

commit 2a41bc2a ("tty: serial: fsl_lpuart: add polled console
functions") caused a build warning about an unused variable, so fix it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Nicolae Rosia <nicolae_rosia@mentor.com>
Cc: Stefan Golinschi <stefan.golinschi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>