usb: gadget: f_fs: add ioctl returning ep descriptor

This patch introduces ioctl named FUNCTIONFS_ENDPOINT_DESC, which
returns endpoint descriptor to userspace. It works only if function
is active.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>