usb: gadget: epautoconf: do not change number of streams

We should not change gadget driver's descriptors just
because we think it's right to do so.

There are several of reasons which would support this
statement but it suffices to say that this was probably
never tested because it updates bmAttributes without
asking the driver if it's ok to do so.

This means that e.g. on UASP gadget it would enable
stream support even for the command endpoint which must
not have stream support enabled.

In fact, this change is fixing the bug introduced by
commit a59d6b91 (usb: gadget: add streams support to
the gadget framework) which was caught when testing
UASP gadget with dwc3 driver.

Cc: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>