igc: add preemptible queue support in mqprio

igc: add preemptible queue support in mqprio

igc already supports enabling MAC Merge for FPE. This patch adds
support for preemptible queues in mqprio.

Tested preemption with mqprio by:
1. Enable FPE:
   ethtool --set-mm enp1s0 pmac-enabled on tx-enabled on verify-enabled on
2. Enable preemptible queue in mqprio:
   mqprio num_tc 4 map 0 1 2 3 0 0 0 0 0 0 0 0 0 0 0 0 \
   queues 1@0 1@1 1@2 1@3 \
   fp P P P E

Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>