bnxt_en: Fix possible crash in bnxt_hwrm_ring_free() under error conditions.

If we encounter errors during open and proceed to clean up,
bnxt_hwrm_ring_free() may crash if the rings we try to free have never
been allocated.  bnxt_cp_ring_for_rx() or bnxt_cp_ring_for_tx()
may reference pointers that have not been allocated.

Fix it by checking for valid fw_ring_id first before calling
bnxt_cp_ring_for_rx() or bnxt_cp_ring_for_tx().

Fixes: 2c61d211 ("bnxt_en: Add helper functions to get firmware CP ring ID.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>