staging: speakup: move those functions which do outgoing serial comms, into serialio.c

This moves spk_synth_immediate and spk_serial_synth_probe functions into
serialio.c. These functions do outgoing serial comms. The move is a step
towards collecting all serial comms in serialio.c. This also renames
spk_synth_immediate to spk_serial_synth_immediate.

Code inside those functions has not been changed. Along the way, this patch
also fixes a couple of spots which were calling spk_synth_immediate directly,
so that the calls now happen via the spk_syth struct.

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>