rc80211_pid_algo.c: remove unused variable declaration

CC [M]  net/mac80211/rc80211_pid_algo.o
net/mac80211/rc80211_pid_algo.c: In function ‘rate_control_pid_rate_init’:
net/mac80211/rc80211_pid_algo.c:304: warning: unused variable ‘si’

Signed-off-by: John W. Linville <linville@tuxdriver.com>