mac80211: further optimise buffer expiry timer

Juuso optimised the timer to not run all the
time in commit 3393a608c4979a94d1887efc05b7.
However, after that it will still run once
more even if all frames just expired. Fixing
that also makes the function return value a
little clearer in the process.

Also, while at it, change the return value
to bool (instead of int).

Cc: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>