brcmfmac: Disable runtime pm for USB.

Currently runtime pm is enabled for USB, but it is not properly
supported by driver. This patch disables the runtime PM support
completely for USB, as it currently can result in problems on
some systems.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>