dt-bindings: net: Correct description of 'broken-turn-around'

The turn around bytes (2) are placed between the control phase of the
MDIO transaction and the data phase, correct the wording to be more
exact.

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