brcmutil: really fix decoding channel info for 160 MHz bandwidth

Previous commit /adding/ support for 160 MHz chanspecs was incomplete.
It didn't set bandwidth info and didn't extract control channel info. As
the result it was also using uninitialized "sb" var.

This change has been tested for two chanspecs found to be reported by
some devices/firmwares:
1) 60/160 (0xee32)
   Before: chnum:50 control_ch_num:36
    After: chnum:50 control_ch_num:60
2) 120/160 (0xed72)
   Before: chnum:114 control_ch_num:100
    After: chnum:114 control_ch_num:120

Fixes: 330994e8

 ("brcmfmac: fix for proper support of 160MHz bandwidth")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>