rc80211-pid: fix definition of rate control interval

Fix the rate control interval definition. Thanks to Mattias Nissler for
spotting this out.

Cc: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>