USB: aircable: rewrite using generic read and write implementations

Kill circular buffers for tx and rx as well as read work thread, and
switch to generic kfifo-based write implementation.

This is an example of how prepare_write_buffer and process_read_urb can
be used to handle protocols with packet headers.

Please note the diffstat which shows that the same functionality is now
provided using only a tenth of the code (including whitespace and
comments, though).
Tested-by: Naranjo, Manuel Francisco <naranjo.manuel@gmail.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>