usb: gadget: atmel_usba: Fix sparse warning

'usba_gadget_template' is used only in this file. Thus, make
'usba_gadget_template' static, in order to fix the following
sparse warning.

warning: symbol 'usba_gadget_template' was not declared. Should it be static?

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>