gtp: Fix inconsistent indenting

Fix the following warning as reported by smatch:

New smatch warnings:
drivers/net/gtp.c:1796 gtp_genl_send_echo_req() warn: inconsistent indenting

Fixes: d33bd757

 ("gtp: Implement GTP echo request")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>