net: phy: smsc: Move SMSC PHY constants to <linux/smscphy.h>

SMSC generation 4 LAN chips integrate an IEEE 802.3 ethernet physical layer.
The ethernet driver for this family of devices needs to access the SMSC PHY
registers and bit-fields.

So, this patch moves these constants to a place where it can be used for both
the PHY and LAN drivers.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: David S. Miller <davem@davemloft.net>