Revert "usb: gadget: Mark usb gsi driver dma memory as cached"

The DMA API attribute flags DMA_ATTR_FORCE_[NON_]COHERENT will
not be supported upstream. USB's usage of this was simply to
ensure that the allocated buffers shared with IPA match in
coherency treatment. This can be done at the entire device
level by ensuring both devices are DMA coherent or not.

This restores the driver to use normal dma_alloc_coherent()
calls throughout.

Change-Id: I9465a74eb56684d71a32c5b121c37a820fc54549
Signed-off-by: Jack Pham <jackp@codeaurora.org>