mac80211: Store sk_pacing_shift in ieee80211_hw

Make it possibly for drivers to adjust the default skb_pacing_shift
by storing it in the hardware struct.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
[adjust commit log, move & adjust comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>