Staging: rspiusb: use NULL virtual address instead of a bogus one

The main problem here is that I just cannot see how this could ever
be correct:

	usb_fill_bulk_urb(pdx->PixelUrb[frameInfo][i], pdx->udev, epAddr,
		(dma_addr_t *) sg_dma_address(&pdx->sgl[frameInfo][i]),
 		sg_dma_len(&pdx->sgl[frameInfo][i]),

You cannot take a DMA address, cast it to a _pointer to_ a DMA address,
and then regard it as a virtual address of the transfer buffer.
However, finding the right virtual address was too hard for me,
so I just stubbed it with NULL. At least usbmon won't oops then
(it will not show any data but it's better than crashing).

Also, too big a buffer was allocated elsewhere.

And since we're at it, drop clearly unnecessary usb_buffer_alloc too,
leaving it where it may be useful.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>