gtp: Add support for checking GTP device type

Add a function that checks if a net device type is GTP.

Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>