ixgbe: replace implicit fall-through comments

DPDK commit message

Convert all "fall-through" comments to actual code. This aligns the code
with the kernel which no longer allows implicit fallthrough.

Signed-off-by: Piotr Skajewski <piotrx.skajewski@intel.com>
Reviewed-by: Alice Michael <alice.michael@intel.com>

Obtained from: DPDK (ae8211f)
MFC after: 1 week