phy: phy-sun4i-usb: Fix optional gpios failing probe

The interrupt 0 is not a valid interrupt number. In the event where the
retrieval of the vbus-det gpio would return null, the gpiod_to_irq
callback would return 0, while the current code makes the assumption
that it is a valid interrupt, and would go on calling request_irq.
Obviously, this would fail, preventing the driver from probing properly,
while the vbus and id gpios are optional.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>