staging/fwserial: Remove bandwidth limit logic

Self-limiting asynchronous bandwidth (via reducing the payload)
is not necessary and does not work, because
 1) asynchronous traffic will absorb all available bandwidth (less that
    being used for isochronous traffic)
 2) isochronous arbitration always wins.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>