serial: imx: Fix build breakage

Fix breakage introduced by
commit c557d392,
'serial: Test for no tx data on tx restart'.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>