net: phy: use ->is_c45 instead of is_10g_interface()

Use phydev->is_c45 instead of is_10g_interface(phydev->interface) to
determine whether clause 45 protocol should be used.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>