brcmfmac: correct reporting HT40 support in wiphy htcap

brcmfmac: correct reporting HT40 support in wiphy htcap

Using 'iw phy' only showed HT20 support in the HT capabilities info.
This patch determines support for HT40 using a firmware query that
is supposed to work for all supported devices.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>