mac80211: mesh gate fixes

Since a v1 of the mesh gate series was accidentally applied, this patch
contains the changes in v2.

These are:
	- automatically make mesh gate a root node.
	- use TU_TO_EXP_TIME macro.
	- initialize timer instead of checking for NULL timer function.
	- cleanups.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>