brcmfmac: cleanup a sizeof()

"flowrings" and "*flowrings" are both pointers so this always returns
sizeof(void *) and the current code works fine.  But "*flowrings" is
intended here and static checkers complain, so lets change it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>