STA OBSS: Add check for overlapping BSSs

STA OBSS: Add check for overlapping BSSs

In the previous implementation connected STA performs OBSS scan
according to requests from its 20/40 MHz AP. However STA checks only 40
MHz intolerance subfield from HT Capabilities element in scan results.
Meanwhile, as per IEEE Std 802.11-2016, 11.16.12, STA should check
overlapping BSSs as well.

Note that all the required code to check overlapping BSSs did already
exist for AP mode since AP does those checks properly before operating
as 20/40 MHz BSS in the 2.4 GHz band. Use that existing code by replace
existing 40 MHz intolerance check in sme_proc_obss_scan() with the new
shared helper function check_bss_coex_40mhz().

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>