ixgbe: Disable DCB and FCoE for X550EM_x and x550em_a

This patch adds IXGBE_FLAG_DCB_CAPABLE flag that is set
for all MACs other than X550EM_x and x550em_a. DCB and
FCoE is disabled for these MACS. DCB initialization
code is moved to a separate function.

Signed-off-by: Usha Ketineni <usha.k.ketineni@intel.com>
Tested-by: Ronald Bynoe <ronald.j.bynoe@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>