bnxt_en: Ring free response from close path should use completion ring

Use completion ring for ring free response from firmware.  The response
will be the last entry in the ring and we can free the ring after getting
the response.  This will guarantee no spurious DMA to freed memory.

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