usb: gadget: dummy_hcd: remove global the_controller variable

The one thing that makes two UDCs+HCDs impossible is the global
the_controller variable. This patch changes this. After device
allocation we allocate the "the_controller" variable and pass it as
platform_data to the UDC and its companion.
We can have now multiple instances dummy hcd and therefore I change the
limit from one to two. I was able to test this with g_ncm adn g_zero:

|# lsusb
|Bus 001 Device 002: ID 0525:a4a0 Netchip Technology, Inc. Linux-USB "Gadget Zero"
|Bus 002 Device 002: ID 0525:a4a1 Netchip Technology, Inc. Linux-USB Ethernet Gadget
|Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
|Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

I was able to start testusb -a and ifconfig usb[01] up with no complains.

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