mac80211: fix sparse complaint in ieee80211_sta_def_wmm_params

A variable 'i' is being shadowed by another one, but the second
one can just be removed.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Vladimir Koutny <vlado@work.ksp.sk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>