net: phy: mscc: rename enum rgmii_rx_clock_delay to rgmii_clock_delay

There is nothing RX-specific about these clock skew values. So remove
"RX" from the name in preparation for the next patch where TX delays are
also going to be configured.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>