USB: skel_read really sucks royally

The read code path of the skeleton driver really sucks

 - skel_read works only for devices which always send data
 - the timeout comes out of thin air
 - it blocks signals for the duration of the timeout
 - it disallows nonblocking IO by design

This patch fixes it by using a real urb, a completion and interruptible waits.

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