usb: dwc2: gadget: Fix DMA memory freeing

Remove DMA memory free from EP disable flow by replacing
dma_alloc_coherent with dmam_alloc_coherent.

Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>