bnxt_en: Keep track of the ring group resource.

Newer firmware will return the ring group resource when we call
hwrm_func_qcaps().  To be compatible with older firmware, use the
number of tx rings as the number of ring groups if the older firmware
returns 0.  When determining how many rx rings we can support, take
the ring group resource in account as well in _bnxt_get_max_rings().
Divide and assign the ring groups to VFs.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>