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>