net80211 / LinuxKPI 802.11: correct enum ieee80211_sta_rx_bw

When moving the enum from LinuxKPI to net80211 it got adjusted to be
used in net80211 style in order to use it with a print_mask (%b).
Turns out that change broke assumptions given the minimum value of
BW_20 no longer was 0. Adjust it back to a plain enum starting at 0
and use an inline function to convert to value names.

Pointy hat to: bz
Fixes: ca389486a9599768e0ba69dca13c208020623083
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48375