net: stmmac: dwmac-sun8i: use return val of readl_poll_timeout()

When readl_poll_timeout() timeout, we'd better directly use its return
value.

Before this patch:
[    2.145528] dwmac-sun8i: probe of 4500000.ethernet failed with error -14

After this patch:
[    2.138520] dwmac-sun8i: probe of 4500000.ethernet failed with error -110

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>