serial: Fix using plain integer instead of Null pointer

Fix build warning that using plain integer as Null pointer.
This is reported by kbuild test robot.

Fixes: ba44dc04

 ("serial: Add Milbeaut serial control")
Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>