brcm80211: fmac: add frame header extension support

A header extension is introduced in the received frame to provide
extra space for dongle information. This won't affect current
supported chipset since the data_offset is 0. But it's necessary for
adding support for future chipset.

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