ibmvnic: set ltb->buff to NULL after freeing

free_long_term_buff() checks ltb->buff to decide whether we have a long
term buffer to free. So set ltb->buff to NULL afer freeing. While here,
also clear ->map_id, fix up some coding style and log an error.

Fixes: 9c4eaabd

 ("Check CRQ command return codes")
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>