net: phy: marvell: Align set_loopback() implementation

Use genphy_loopback() to disable loopback like ksz9031_set_loopback().
This way disable loopback is implemented only once within
genphy_loopback() and the set_loopback() implementations look similar.

Also fix comment about msleep() in the out-of loopback case which is not
executed in the out-of loopback case.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250312203010.47429-5-gerhard@engleder-embedded.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>