USB: opticon: switch to generic read implementation

Switch to the more efficient generic read implementation.

Note that the generic implementation is not required to hold the tty
port mutex during resume due to the read-urb free mask and write start
flag.

Note also that the generic resume implementation will call generic
write start if there is a bulk-out end-point, but that nothing will be
submitted as the write fifo is not used and is empty.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>