brcmfmac: make sdio suspend wait for threads to freeze

Borrowed the idea of the PM freezer to make sdio suspend wait for
watchdog and DPC thread to freeze at a safe point in their thread
routine. The suspend takes 20-25 msec.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>