usb: gadget: atmel: fix transfer of queued requests

In the existing implementation, multiple requests queued up on an
endpoint are subject to getting evicted without transmission.

For both control and bulk endpoints, their respective logic found in
usba_control_irq()/usba_ep_irq() guarantees that TX FIFO is empty before
data is sent out, and that request_complete() gets called once the
transaction has been finished. At this point however, if any additional
requests are found on the endpoint queue, they will be processed by
submit_next_request(), which makes no checks against the above
conditions, trashing data on a busy FIFO and neglecting completion
handlers.

Fix the above issues by removing the calls to submit_next_request(),
and thus forcing the pending requests to be processed on the next pass
of the respective endpoint logic. While at it, remove a DBG message, as
that branch becomes part of regular flow.

This restores mass storage mode operation on Microchip ATSAMA5D27 SoC.

Signed-off-by: Artur Rojek <artur@conclusive.pl>