USB: O_NONBLOCK in read path of skeleton

Non blocking IO is supported in the read path of usb-skeleton.
This is done by just not blocking. As support for handling signals
without stopping IO is already there, it can be used for O_NONBLOCK, too.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>