gtp: uapi: fix GTPA_MAX

Subtract one to __GTPA_MAX, otherwise GTPA_MAX is off by 2.

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>