cfg80211: Allow reassociation to be requested with internal SME

If the user space issues a NL80211_CMD_CONNECT with
NL80211_ATTR_PREV_BSSID when there is already a connection, allow this
to proceed as a reassociation instead of rejecting the new connect
command with EALREADY.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
[validate prev_bssid]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>