Staging: quatech_usb2: write_room rewrite

The following patch re-writes the write_room method to work in the
obvious sort of way, so it keeps it's promises to the layers above.
Based on feedback from Alan Cox.

Signed-off-by: Richard Ash <richard@audacityteam.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>