staging: speakup: add spk_io_ops struct to spk_synth

staging: speakup: add spk_io_ops struct to spk_synth

This patch adds spk_io_ops struct which contain those methods whose job is to
communicate with synth device. Currently, all comms with external synth
device use raw serial i/o. The idea is to group all methods which do the
actual communication with external device into this new struct. Then migrating
a serial-based synth over to an alternative to raw serial i/o will mean
swapping serial spk_io_ops instance with the io_ops instance of the new
method, making the migration simpler.

At the moment, this struct only contains one method, synth_out but more will
be added in future when migrating synths which require input functionality.
Also at the moment, synth_out method has one implementation which uses
serial i/o. Plan is to add an alternative.

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>