usb: gadget: at91: using USBA_NR_DMAS for DMA channels

The SoCs earlier than sama5d3, they have the same number endpoints
and DMA channels. In driver code, they use the same definition
USBA_NR_ENDPOINTS for both endpoints and dma channels. However,
in sama5d3, it has different number for endpoints and DMA channels.
So, define a new macro USBA_NR_DMAs for DMA channels. And the
USBA_NR_ENDPOINS is not used anymore, remove it at the same time.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>