staging: ion: debugfs invalid gfp mask

The current code attempts assignment of -1 to an unsigned type. Note that
in a downstream function ion_page_pool_shrink this mask is only ever
evaluated against __GFP_HIGHMEM
(drivers/staging/android/ion/ion_page_pool.c, line 125).

Signed-off-by: Derek Yerger <dy@drexel.edu>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>