bnxt_en: Clean up completion ring page arrays completely

We recently changed the completion ring page arrays to be dynamically
allocated to better support the expanded range of ring depths.  The
cleanup path for this was not quite complete.  It might cause the
shutdown path to crash if we need to abort before the completion ring
arrays have been allocated and initialized.

Fix it by initializing the ring_mem->pg_arr to NULL after freeing the
completion ring page array.  Add a check in bnxt_free_ring() to skip
referencing the rmem->pg_arr if it is NULL.

Fixes: 03c74487 ("bnxt_en: Don't use static arrays for completion ring pages")
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>