staging: speakup: fix synth caching when synth init fails

synths[] array caches currently loaded synths. synth_add checks
synths[] before adding a new one. It however ignores the result of
do_synth_init. So when do_synth_init fails, the failed synth is still
cached. Since, as a result module loading fails too, synth_remove -
which is responsible for removing the cached synth - is never called.
Next time the failing synth is added again it succeeds because
synth_add finds it cached inside synths[].

This patch fixes this by caching a synth only after do_synth_init
succeeds.

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>