iwlwifi: check kasprintf() return value

kasprintf() can fail, we should check the return value.

Fixes: 5ed540ae ("iwlwifi: use mac80211 throughput trigger")
Fixes: 8ca151b5 ("iwlwifi: add the MVM driver")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>