mac80211: rc80211_pid eliminate sparse warnings

This patch eliminates sparse warnings in pid rate scale algorithm
'debugfs: allow access to signed values' patch hit the dead end
year ago w/o much echo so I guess there is no real need to address this
properly.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>