IB/mlx5: Fix GRE flow specification

Currently the driver sets the mask of the gre_protocol to 0xffff
without consideration in the user request.

Fix it by copy the mask from the verbs spec.

Fixes: da2f22ae ("IB/mlx5: Add support for GRE flow specification")
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>