net/mlx5: Check max encap header size capability

Instead of comparing to a const value, check the value of max encap
header size capability as reported by the Firmware.

Fixes: 575ddf58

 ('net/mlx5: Introduce alloc_encap and dealloc_encap commands')
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>