ixgbe: Fix PFC mask generation

Fix PFC mask generation to OR in only a single bit for each priority in
the PFC mask returned via netlink.

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>