ixgbe: refactor initialization of feature flags

This patch reshuffles the switch/case structure of the flag assignment to
allow for the flags to be set for each MAC type separately. This is needed
for new HW that does not have feature parity with older HW.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Jack Morgan <jack.morgan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>