usb: gadget: add 'quirk_stall_not_supp' to usb_gadget

Due to some UDC controllers may not support stalling, usb gadget layer
needs to provide a generic way to inform gadget functions about non-standard
hardware limitations.

This patch adds 'quirk_stall_not_supp' field to struct usb_gadget and helper
function gadget_is_stall_supported(). It also sets 'quirk_stall_not_supp'
to 1 in at91_udc driver, which has such limitation.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>