Fix last_sample initialization. kzalloc'ing the per-STA data wasn't enough,
as jiffies can assume negative values as well. This fixes a bug which
prevented correct PID operation for a while after booting.

Thanks to Michael Buesch for reporting this.
Reported-and-tested-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>