ixgbe: add /* fallthrough */ comment to case statements

This semicomplex switch-case has various fallthrough portions, that were
not indicated by a /* fallthrough */ comment.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>