brcmfmac: Fix bug in flowring management.

The hash index stored in the flowrings is of type u16 but gets
stored in u8. This can result in incorrect indexing and possibly
result in crashes. This patch fixes the type.

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>