USB: opticon: Fix long-standing bugs in opticon driver

The bulk-read callback had two bugs:
a) The bulk-in packet's leading two zeros were returned (and the two last
   bytes truncated)
b) The wrong URB was transmitted for the second (and later) read requests,
   causing further reads to return the entire packet (including leading
   zeros)

Signed-off-by: Alon Ziv <alon-git@nolaviz.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>