drivers: net: cpsw-phy-sel: add dev_warn() for unsupported PHY mode

The cpsw-phy-sel driver supports only MII, RMII, and RGMII PHY modes,
and silently handled any other values as if MII was specified. In a
case where the PHY mode was incorrectly specified, or a bug elsewhere,
there would be no indication of a problem. If MII was the correct mode,
then this will go unnoticed, otherwise the symptom will be a failure
to transmit/receive data over the RMII/RGMII link.

Add a dev_warn() to make this condition obvious and provide a
breadcrumb to follow.

Cc: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David Rivshin <drivshin@allworx.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>