nl80211: Move duplicate scan result removal to bss.c

nl80211: Move duplicate scan result removal to bss.c

The way the removal of duplicated (one per frequency) BSS entries in the
cfg80211 scan results were removed in driver_nl80211_scan.c
bss_info_handler() depended on having the full scan results available to
allow iteration through the other entries. This is problematic for the
goal of being able to optimize memory allocations for scan result
fetching in a manner that would not build the full result buffer in
memory.

Move this duplicate removal into bss.c since it has sufficient
information available for doing the same determination of which one of
two BSS entries is more current.

Signed-off-by: Jouni Malinen <j@w1.fi>