serial: imx: Remove unused 'tty' variable

Commit 2e124b4a (TTY: switch tty_flip_buffer_push) introduced the following
build warning:

drivers/tty/serial/imx.c:519:21: warning: unused variable 'tty' [-Wunused-variable]

Remove the unused 'tty' variable.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>