Staging: speakup: Use module_spk_synth

Macro module_spk_synth can be used for speakup drivers
whose init and exit paths does only module registrations.
So, here remove some boilerplate code by using
module_spk_synth.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>