net80211 / LinuxKPI 802.11: use enum ieee80211_sta_rx_bw for ni_chw

net80211 node ni_chw currently encodes the channel width as Mhz number.
LinuxKPI 802.11 uses enum ieee80211_sta_rx_bw for the same.

Rather than keeping the "20" and "40" throughout the code (eventually
expanded to 80/160/320) switch them over to use the enum throughout.

Once we get to 320Mhz channel widths we would otherwise also need to
extend the uint8_t in struct ieee80211_node; making
enum ieee80211_sta_rx_bw __packed allows us for lots more channel
widths without breaking the KBI.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D47891

(cherry picked from commit ca389486a9599768e0ba69dca13c208020623083)
(cherry picked from commit 2c8b0d6205f6f98855773e3a82640b50abb2f2f6)