usb: atmel_usba_udc: fix it to deal with final dma channel

As, the interrupt for DMA is counted from 1, so need to checked
the USBA_NR_DMAS, in old way, it only check (USBA_NR_DMAS - 1),
so fix it.

Reported-by: Max Liao <liaops@embest-tech.com>
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>