net/mlx5e: Use correct return type

modify_header_match_supported() should return type bool but
it returns the value returned by is_action_keys_supported()
which is type int.

is_action_keys_supported() always returns either -EOPNOTSUPP
or 0 and it shouldn't change as the purpose of the function
is checking for support. so just make the function return
a bool type.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>