nfp: flower: fix incorrect struct type in GRE key_size

Looks like a copy-paste error sneaked in here at some point,
causing the key_size for these tunnels to be calculated
incorrectly. This size ends up being send to the firmware,
causing unexpected behaviour in some cases.

Fixes: 78a722af

 ("nfp: flower: compile match for IPv6 tunnels")
Reported-by: Chaoyong He <chaoyong.he@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20221007092132.218386-1-simon.horman@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>