brcmfmac: clear events in brcmf_fweh_detach() will always fail

Clearing firmware events in brcmf_fweh_detach() is always failing
because it is called only upon driver remove and communication
with firmware is no longer possible.

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>