bwn(4): migrate bwn(4) to the native bhnd(9) interface, and drop siba_bwn.
- Remove the shim interface that allowed bwn(4) to use either siba_bwn or bhnd(4), replacing all siba_bwn calls with their bhnd(4) bus equivalents.
- Drop the legay, now-unused siba_bwn bus driver.
- Clean up bhnd(4) board flag defines referenced by bwn(4).
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13518