linuxkpi: Improve LKPI_80211_TRACE_MO message

The message used to have a prefix of the function and line, followed by
:_ and the supplied message. Commit 3206587a20de ("LinuxKPI: 802.11:
improve scan handling") changed the prefix to add the CPU, tid, and
ticks. _ and the caller-supplied message were joined to the ticks
value, resulting in a somewhat odd message. Instead use : to
separate them.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50667