ixgbe: cleanup ixgbe_set_tx_csum ethtool flags configuration

This change makes it so that we always disable SCTP regardless of mac type
since we shouldn't need to check mac type before disabling a feature that
isn't supported on a given piece of hardware.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>