usb: chipidea: udc: improve error handling on _hardware_enqueue

_hardware_enqueue() didn't check for errors when using
add_td_to_list() which can fail if dma_pool_alloc fails, thus
causing a kernel panic when lastnode->ptr is NULL.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>