mac80211: Remove unused 'i' variable

Commit 5bcae31d (mac80211: implement multi-vif in-place reservations)
introduced ieee80211_vif_use_reserved_switch() with a counter variable
'i' that is set but not used. Compiling with W=1 gives the following
warning, fix it.
net/mac80211/chan.c: In function ‘ieee80211_vif_use_reserved_switch’:
net/mac80211/chan.c:1273:6: warning: variable ‘i’ set but not used [-Wunused-but-set-variable]

This is a harmless warning, and is only being fixed to reduce the
noise obtained with W=1 in the kernel.

Fixes: 5bcae31d ("mac80211: implement multi-vif in-place reservations")
Cc: Michal Kazior <michal.kazior@tieto.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kirtika Ruchandani <kirtika@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>