brcmfmac: assure device is ready for download after brcmf_chip_attach()

Make the brcmf_chip_attach() function responsible for putting the
device in a state where it is accessible for firmware download.

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: Kalle Valo <kvalo@codeaurora.org>