brcmfmac: call brcmf_attach() just before calling brcmf_bus_started()

Now we can move brcmf_attach() until after the firmware has been downloaded
to the device. Make the call just before brcmf_bus_started().

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.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>