ixgbe: driver fix for link flap

Fix up code so that changes in DCB settings
are detected only when ixgbe_dcbnl_set_all is called.
Previously, a series of 'change' commands followed by
a call to ixgbe_dcbnl_set_all() would always be handled
as a HW change - even if the net change was zero.
This patch checks for this case of no actual change and
skips going through the HW set process.

Without this fix, the link could reset and result in
a link flap.

The core change in this patch is to check for changes
in the ixgbe_copy_dcb_cfg() routine - and return
a bitmask of detected changes.  The other
places where changes were detected previously can be removed.

Signed-off-by: Eric Multanen <eric.w.multanen@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>