USB: safe_serial: reimplement write using generic framework

Kill custom single-urb write implementation.

Note that this driver still depended on the write callback from the old
generic framework.

Tested against original read processing using a cp210x device in a
loopback setup.

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