usb: chipidea: udc: Use dma_pool_zalloc()

We can make the code simpler by using dma_pool_zalloc() instead
of calling dma_pool_alloc() and then a memset().

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>