net: mdio-ipq4019: add delay after clock enable

Experimentation shows that PHY detect might fail when the code attempts
MDIO bus read immediately after clock enable. Add delay to stabilize the
clock before bus access.

PHY detect failure started to show after commit 7590fc6f ("net:
mdio: Demote probed message to debug print") that removed coincidental
delay between clock enable and bus access.

10ms is meant to match the time it take to send the probed message over
UART at 115200 bps. This might be a far overshoot.

Fixes: 23a890d4

 ("net: mdio: Add the reset function for IPQ MDIO driver")
Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>