staging: speakup: spk_serial_out and spk_wait_for_xmitr to take synth arg

These two functions are always called from a context where spk_synth instance
is available. They also use the spk_synth instance but instead of taking it
as an argument, they rely on a global spk_synth instance inside synth.c which
points to the same synth as the one being passed in as argument.

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>