usb: gadget: f_fs: Pass along set_halt errors.

Users can apply i/o in the wrong direction on an
endpoint to stall it. In case there is an error
that does not allow the endpoint to be stalled,
we want the user to know.

An operation to stall the endpoint will return
EBADMSG if successful, EAGAIN if there are still
queued requests, and other errors depending on
the underlying implementation.

Also remove the conditional since it is always true.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Jerry Zhang <zhangjerry@google.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>