Revert "brcmfmac: fix NULL pointer derefence during USB disconnect"

This reverts commit 5cdb0ef6. Subsequent
changes make rework the driver code fixing the issue differently.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>