mac80211: allow drivers to set max MTU

Make it possibly for drivers to adjust the default max_mtu
by storing it in the hardware struct and using that value
for all interfaces.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Link: https://lore.kernel.org/r/1567738137-31748-1-git-send-email-wgong@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>