brcmfmac: drop brcmf_bus_detach and inline its code

Driver used to call brcmf_bus_detach only from one place and it already
contained a check for drvr not being NULL. We can get rid of this extra
function, call brcmf_bus_stop directly and simplify the code.
There also isn't brcmf_bus_attach function which one could expect so it
looks more consistent this way.

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>