brcmfmac: rename brcmf_bus_start function to brcmf_bus_started

This intends to make init/attach process slightly easier to follow.

What driver was doing in brcmf_bus_start wasn't bus specific at all and
function brcmf_bus_stop wasn't undoing things done there. This function
is supposed to be called by bus specific code when the bus is ready.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>