ixgbe: fix RAR entry counting for generic and fdb_add()

Do RAR entry accounting correctly so that errors are reported and
promisc mode is set correctly when the number of entries exceeds
the hardware limits.

This can happen with many macvlan devices attached to the PF or
by adding many fdb entries in SR-IOV modes.

Also this includes a small refactor to fdb_add() to avoid having so
many nested if/else statements after adding a check for the number
or RAR entries.

The max entries for the PF is currently 16 we allow 15 additional
entries to account for the defined MAC.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>