dsa: lan9303: move Turbo Mode bit init

In preparing to remove the .adjust_link api, I am moving the one-time
initialization of the device's Turbo Mode bit into a different execution
path. This code clears (disables) the Turbo Mode bit which is never used
by this driver. Turbo Mode is a non-standard mode that would allow the
100Mbps RMII interface to run at 200Mbps.

Signed-off-by: Jerry Ray <jerry.ray@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>