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