net: dsa: mv88e6xxx: Longer timeout for PTP TX timestamp

For slow processors using bit-banging MDIO, 20ms can be too short a
timeout when waiting for the transmit timestamp to become
available. Double it to 40ms.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>