ixgbe: fix setting of promisc mode when using mac-vlans

Introduce uc_set_promisc flag to fix enabling of promisc mode
when exceeding the number of supported RAR entries.

Issue discovered by Ben Greear when using mac-vlans.

Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>