usb: gadget: udc: atmel: remove outdated comment in usba_ep_disable()

Fixed commit removed the offending behaviour from the driver, but missed
the comment and associated test. Remove them now.

Fixes: 38e58986

 ("usb: gadget: udc: atmel: don't disable enpdoints we don't own")
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Felipe Balbi <balbi@kernel.org>