ice: Fix the calculation of ICE_MAX_MTU

Currently ICE_MAX_MTU subtracts only ETH_HLEN from max frame size and
adds ETH_FCS_LEN and VLAN_HLEN, which is not what was intended.
The ETH_HLEN + ETH_FCS_LEN + VLAN_HLEN expression should be surrounded
with parentheses.

Wrap mentioned expression and take into account VLAN double tagging.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>