usb: dwc3: core: write LINUX_VERSION_CODE to our GUID register

DWC3's GUID register is supposed to be used to write
any sort of version we might want. It helps when getting
bug reports for platforms you don't have HW to know
which kernel version of the driver was running on the
platform.

Because we don't really track driver version, but we _do_
track the kernel version, let's write LINUX_VERSION_CODE to
that register and use it for debugging.

Reviewed-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>