bwn(4): Enable, by default, the opt-in support for bhnd(4) introduced in
r326454.

bwn(4)/bhnd(4) has been tested with most chipsets currently supported by
bwn(4), and this change should be transparent to existing bwn(4) users;
please report any regressions that you do encounter.

To revert to using siba_bwn(4) instead of bhnd(4), place the following
lines in loader.conf(5):

hw.bwn_pci.preferred="0"

Once we're satisfied that the switch to bhnd(4) has seen sufficient broader
testing, bwn(4) will be migrated to use the native bhnd(9) interface
directly, and support for siba_bwn(4) will be dropped (see D13518).

Sponsored by: The FreeBSD Foundation