net: phy: Optimize phy speed mask to be compatible to yt8821

yt8521 and yt8531s as Gigabit transceiver use bit15:14(bit9 reserved
default 0) as phy speed mask, yt8821 as 2.5G transceiver uses bit9 bit15:14
as phy speed mask.

Be compatible to yt8821, reform phy speed mask and phy speed macro.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Frank Sae <Frank.Sae@motor-comm.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>