brcmfmac: rename chip download functions

The functions brcmf_chip_[enter/exit]_download() are not exclusively
used for firmware download so rename these more appropriate.

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>