net: phy: bcm84881: use phy_read_mmd_poll_timeout() to simplify the code

use phy_read_mmd_poll_timeout() to replace the poll codes for
simplify bcm84881_wait_init() function.

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