mac80211_hwsim: fix incorrect dev_alloc_name failure goto

If dev_alloc_name fails, hwsim_mon's memory allocated in alloc_netdev
needs to be freed.
Change goto command in dev_alloc_name failure to out_free_mon in
order to perform free_netdev.

Signed-off-by: Michael Vassernis <michael.vassernis@tandemg.com>
Link: https://lore.kernel.org/r/20191003073049.3760-1-michael.vassernis@tandemg.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>