brcm80211: extend channel conversion functions for 80MHz support

The channel values used by firmware is handled using conversion functions
depending on the type of chip. These functions were already in place but
lacked proper support for 80MHz channel definitions. This patch adds the
support for that.

Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@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>