iwlwifi: fix async station command crash

Before Emmanuel's change to use a copy of the command
("iwlwifi: get the correct HCMD in the response handler")
the iwl_add_sta_callback() function would have used a
random pointer to somewhere when processing responses
to an async command, while that wasn't valid data it
was at least a valid pointer. Now, the pointer will be
NULL in this case, thus crashing.

Fix this by exiting the function early if no command
is passed back which means it was sent asynchronously.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>