Currently queues are stopped when their length reaches their length limit,
but are restarted only when the size of freed range of packet buffer is
not less than the size of the largest possible packet.

This causes permanent queue stop on radio visibility loss in the middle
of ping series: there is plenty of room in the packet buffer, but it is
never freed more than 3 (size of 'best effort' queue) * 288 (ping packet
plus headers) bytes at once.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>