usb: gadget: atmel: fix build warning

This patch fixes the following build warning:

drivers/usb/gadget/udc/atmel_usba_udc.c:707:2: warning: format ‘%x’
expects argument of type ‘unsigned int’, but argument 4 has type
‘dma_addr_t’ [-Wformat=]

Signed-off-by: Felipe Balbi <balbi@ti.com>