phy: mvebu-cp110-comphy: explicitly set the pipe selector

The pipe selector is used to select some modes (such as USB or PCIe).
Otherwise it must be set to 0 (or "unconnected"). This patch does this
to ensure it is not set to an incompatible value when using the
supported modes (SGMII, 10GKR).

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>