staging: speakup: move spk_stop_serial_interrupt into synth-specific release function

This moves call to spk_stop_serial_interrupt() function out of synth_release()
and into release() method of specific spk_synth instances. This is because
the spk_stop_serial_interrupt() call is specific to current serial i/o
implementation. Moving it into each synth's release() method gives the
decision of calling  spk_stop_serial_interrupt() to that synth.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>