net/mlx5e: Fix outer_header_zero() check size

outer_header_zero() routine checks if the outer_headers match of a
flow-table entry are all zero.

This function uses the size of whole fte_match_param, instead of just
the outer_headers member, causing failure to detect all-zeros if
any other members of the fte_match_param are non-zero.

Use the correct size for zero check.

Fixes: 6dc6071c ("net/mlx5e: Add ethtool flow steering support")
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>