net: phy: Add phy_sysfs_create_links helper function

This is a cosmetic patch that wraps the operation of creating sysfs
links between the netdev->phydev and the phydev->attached_dev.

This is needed to keep the indentation level in check in a follow-up
patch where this function will be guarded against the existence of a
phydev->attached_dev.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>