usb: udc: core: Error if req->buf is either from vmalloc or stack

Check that req->buf is a valid DMA capable address, produce a warning
and return an error if it's either coming from vmalloc space or is an on
stack buffer.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>