USB: dummy-hcd: use configurable endpoint naming scheme

USB gadget subsystem uses the following naming convention for UDC
endpoints:

- "ep-a" names for fully configurable endpoints (address, direction and
  transfer type can be changed);

- "ep1in", "ep12out-bulk" names for fixed function endpoints (fixed
  address, direction and/or transfer type).

Dummy UDC endpoints are capable of full configuration, but named using
the second scheme.

This patch changes the names of generic Dummy UDC endpoints to "ep-aout",
"ep-bin", etc., to advertise that they have configurable addresses and
transfer types (except that Dummy UDC doesn't support ISO transfers), but
fixed direction.

This is required for Raw Gadget (and perhaps for some other drivers),
that reasons about whether an endpoint has configurable address based
on its name.

Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>