staging/ft1000: fix sparse warning

sparse warns about having 0 assigned to a pointer,
fix it up by using NULL.

The following sparse warning is fixed

drivers/staging/ft1000/ft1000-usb/ft1000_debug.c:170:52: warning: Using plain integer as NULL pointer

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>