mac80211: simplify mesh code

Doing mod_timer() conditionaly is easier than conditionally unlocking
and jumping around...

Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Acked-by: Linus Lüssing <ll@simonwunderlich.de>
Link: https://lore.kernel.org/r/20200604214157.GA9737@amd


Signed-off-by: Johannes Berg <johannes.berg@intel.com>