staging: gasket: fix plain integer as NULL pointer warning

Trivial fix to remove sparse warnings:

  drivers/staging/gasket/gasket_page_table.c:884:40: warning: Using plain integer as NULL pointer
  drivers/staging/gasket/gasket_page_table.c:1743:57: warning: Using plain integer as NULL pointer
  drivers/staging/gasket/gasket_page_table.c:1768:57: warning: Using plain integer as NULL pointer

Signed-off-by: Ivan Bornyakov <brnkv.i1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>