mac80211: Remove unused 'len' variable

Commit 633e2713 (mac80211: split sched scan IEs) introduced the
len variable to keep track of the return value of
ieee80211_build_preq_ies() but did not use it. Compiling with W=1
gives the following warning, fix it.

net/mac80211/scan.c: In function ‘__ieee80211_request_sched_scan_start’:
net/mac80211/scan.c:1123:9: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]

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

Fixes: 633e2713 ("mac80211: split sched scan IEs")
Cc: David Spinadel <david.spinadel@intel.com>
Cc: Alexander Bondar <alexander.bondar@intel.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>