net80211: scan/internal: change boolean argument from int to bool

ieee80211_probe_curchan() passes a "force" argument which is bool.
Make it such. Adjust the (*sc_scan_probe_curchan)() KPI to bool
as well. This is all a big NOP as the only implementor of this
function, ieee80211_swscan_probe_curchan(), does not use the argument
at all.

I came across this when pondering a different scan implementation.
Rather than dropping the change remove the argument from the function,
and push the cleanup out given it is purely net80211 internal code
(the argument may have reason for existance in the future).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45816