net: ethernet: mediatek: ppe: fix busy wait loop

The intention is for the loop to timeout if the body does not succeed.
The current logic calls time_is_before_jiffies(timeout) which is false
until after the timeout, so the loop body never executes.

Fix by using readl_poll_timeout as a more standard and less error-prone
solution.

Fixes: ba37b7ca

 ("net: ethernet: mtk_eth_soc: add support for initializing the PPE")
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>