bnxt_en: Fix NULL pointer dereference in a failure path during open.

If bnxt_hwrm_ring_free() is called during a failure path in bnxt_open(),
it is possible that the completion rings have not been allocated yet.
In that case, the completion doorbell has not been initialized, and
calling bnxt_disable_int() will crash.  Fix it by checking that the
completion ring has been initialized before writing to the completion
ring doorbell.

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