usb: gadget: FunctionFS: use consistent naming with regard to ffs_lock

Consistently prefix function name with underscore if the function has to
be called with ffs_lock taken.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>