usb: phy: Fix NULL pointer exception during usb_get_phy

Upon initialisation (driver probe) a NULL pointer exception
is triggered. This is due to lack of initialisation of
device field in phy structure, which is used by phy
framework in usb_get_phy().

Fix it by initialising the device field.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>