usb: kbd: Also accept keyboards with Interrupt OUT endpoint

The OUT endpoint can just be ignored as it is not used, just as the
corresponding Set_Report request for IN-only interfaces. E.g. the
Linux gadget hid keyboard also provides an interrupt endpoint.

Also cleanup confusing debug messages like "found set protocol", which
is printed when a keyboard device is found, while the Set_Protocol request
is issued quite some time later.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>