usb: dwc3: Use generic_phy_valid() helper

The documentation for struct phy state that "The content of the
structure is managed solely by the PHY API and PHY drivers".

Change to use the generic_phy_valid() helper to check if phy is valid.
Also remove setting phy->dev to NULL now that generic_phy_get_by_name()
properly initialize phy->dev to NULL.

Fixes: 142d50fb ("usb: dwc3: Add support for usb3-phy PHY configuration")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>