brcmfmac: remove worker from .ndo_set_mac_address() callback

As it turns out there is no need to use a worker for the callback
because it is not called from atomic context.

Reported-by: Dan Williams <dcbw@redhat.com>
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>