net: phy: mdio-mux-bcm-iproc: use readl_poll_timeout() to simplify code

use readl_poll_timeout() to replace the poll codes for simplify
iproc_mdio_wait_for_idle() function

Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>