net: phy: mscc: enable VSC8501/2 RGMII RX clock

By default the VSC8501 and VSC8502 RGMII/GMII/MII RX_CLK output is
disabled. To allow packet forwarding towards the MAC it needs to be
enabled.

For other PHYs supported by this driver the clock output is enabled
by default.

Fixes: d3169863 ("net: phy: mscc: add support for VSC8502")
Signed-off-by: David Epping <david.epping@missinglinkelectronics.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>