cfg80211: clean up variable use in cfg80211_parse_colocated_ap()

The "ap_info->tbtt_info_len" and "length" variables are the same value
but it is confusing how the names are mixed up.  Let's use "length"
everywhere for consistency.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YJaMNzZENkYFAYQX@mwanda


Signed-off-by: Johannes Berg <johannes.berg@intel.com>