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

commit a816e3113b63753c330ca4751ea1d208e93e3015 upstream.

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>
Cc: Lotfi Manseur <lotfi.manseur@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>