net/bnxt: fix double allocation of ring groups

After commit "d68249f88266", driver allocates ring groups in
bnxt_alloc_hwrm_rx_ring(). But during port start, driver invokes
bnxt_alloc_hwrm_rx_ring() followed by bnxt_alloc_all_hwrm_ring_grps().
This will cause the FW command failure in bnxt_alloc_all_hwrm_ring_grps()

To fix this, just don't create the ring group if it is already created.

Fixes: 9b63c6fd70e3 ("net/bnxt: support Rx/Tx queue start/stop")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Lance Richardson <lance.richardson@broadcom.com>