usb: gadget: dummy_hdc: prepare for multiple instances

This patch replaces the single pdev variable by an array. One change:
The device id is no longer -1 (i.e. none) but 0.
This is prepation work for multiple instances of the dummy_hcd + udc
which should help to test gadget framework with multiple UDCs.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>