usb: dwc3: switch to GPL v2 only

This is a Linux-only driver which makes use
of GPL-only symbols. It makes no sense to
maintain Dual BSD/GPL licensing for this driver.

Considering that the amount of work to use this
driver in any different operating system would likely
be as large as developing the driver from scratch and
considering that we depend on GPL-only symbols, we
will switch over to a GPL v2-only license.

Cc: Anton Tikhomirov <av.tikhomirov@samsung.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>