Staging: quatech_usb2: chars_in_buffer() fix

It seems I can't count.

Patch to fix chars_in_buffer() method (broken in vendor driver), and
subsequently simplify the close() method, making it somewhat less
broken. There is quite a bit more improvement to do on this method
though.

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