USB: ftdi_sio: Quiet sparse noise about using plain integer was NULL pointer

Pointers should not be compared to plain integers.
Quiets the sparse warning:
warning: Using plain integer as NULL pointer

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>