usb: dwc_otg: Use dma allocation for mphi dummy_send buffer

usb: dwc_otg: Use dma allocation for mphi dummy_send buffer

The FIQ driver used a kzalloc'ed buffer for dummy_send,
passing a kernel virtual address to the hardware block.
The buffer is only ever used for a dummy read, so it
should be harmless, but there is the chance that it will
cause exceptions.

Use a dma allocation so that we have a genuine bus address,
and read from that.
Free the allocation when done for good measure.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>