Turn ieee80211_start_scan_locked() into a swscan module private method.

It turns out that ieee80211_start_scan_locked() is only ever called by
the swscan code and it won't likely be required by firmware scanning
implementations.

So, don't bother keeping it in ieee80211_scan.c and it likely won't
become an API call.

Tested:

• Intel 5100, STA mode
• AR5416, STA mode