net: phy: realtek: use the BIT and GENMASK macros

This makes it easier to compare the #defines with the datasheets.
No functional changes.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>