staging:wlags49_h2: Fix sparse warnings in sta_h25.c

This patch fixes sparse warnings "Using plain integer as NULL pointer" in
sta_h25.c

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Reviewed-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>