usb: dwc3: gadget: use generic map/unmap routines

those routines have everything we need to map/unmap
USB requests and it's better to use them.

In order to achieve that, we had to add a simple
change on how we allocate and use our setup buffer;
we cannot allocate it from coherent anymore otherwise
the generic map/unmap routines won't be able to easily
know that the GetStatus request already has a DMA
address.

Signed-off-by: Felipe Balbi <balbi@ti.com>