net: lan966x: fix kernel oops on ioctl when I/F is down

ioctls handled by phy_mii_ioctl() will cause a kernel oops when the
interface is down. Fix it by making sure there is a PHY attached.

Fixes: 735fec99 ("net: lan966x: Implement SIOCSHWTSTAMP and SIOCGHWTSTAMP")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220328220350.3118969-1-michael@walle.cc


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