brcmfmac: create common function for handling brcmf_proto_hdrpull()

In receive path brcmf_proto_hdrpull() needs to be called and handled
similar in brcmf_rx_frame() and brcmf_rx_event(). Move that duplicated
code in separate function.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>