usb: phy: move some error messages to debug

the PHY layer is supposed to be optional,
considering some PHY have no control bus
for SW to poke around.

After commit 1ae5799e (usb: hcd: Initialize
USB phy if needed) any HCD which didn't provide
a PHY driver would emit annoying error messages.

In this patch we're decreasing those messages
to debugging only and we also add a PHY prefix
or use dev_dbg so we know where they're coming from.

Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>