brcmfmac: remove code and comment for older kernel support

In the code of the receive path some code was dealing with how
things were done in older kernels. Not really needed for an
upstream driver.

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>