ixgbe: Correct check for change in FCoE priority

Correct a check for change in FCoE priority when IEEE mode DCB is in use.
In IEEE mode a different function has to be used to get the FCoE priority
mask. Also, the check for the mask assumed that only one priority was set.
In case there should be more than one, check just the bit.

These changes help avoid link flapping issues that can come up when IEEE
DCB is in use.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>