wifi: nl80211: acquire wdev mutex earlier in start_ap

We need to hold the wdev mutex already in order to call
nl80211_parse_tx_bitrate_mask(), so acquire it earlier.

Fixes: 7b0a0e3c

 ("wifi: cfg80211: do some rework towards MLO link APIs")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>