net: ethernet: octeon_mgmt: Account for second possible VLAN header

Octeon's input ring-buffer entry has 14 bits-wide size field, so to account
for second possible VLAN header max_mtu must be further reduced.

Fixes: 109cc165

 ("ethernet/cavium: use core min/max MTU checking")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>