usb: gadget: atmel_usba: Use devm_*() functions

Use devm_*() functions to make cleanup paths simpler.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>