usb: gadget: mv_udc: avoid sleeping on spinlock

build_dtd() can be called when hold a spinlock, but GFP_KERNEL may cause
dma_pool_alloc() sleep, So we need use GFP_ATOMIC instead of GFP_KERNEL.
But using GFP_ATOMIC may cause failure when allocating memory, add error
handler to handle it.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>