ixgbe: Use 1TC DCB instead of disabling DCB for MSI and legacy interrupts

This change makes it so that we can use 1TC DCB in the case of MSI and
legacy interrupts.  The advantage to this is that it allows us to fully
support FCoE w/ DCB instead of having to drop to link flow control only
when using these interrupt modes.

Cc: John Fastabend <john.r.fastabend@intel.com>
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>