staging: speakup: refactor synths array to use a list

The synths[] array is a collection of synths acting like a list.
There is no need for synths to be an array, so refactor synths[] to use
standard kernel list_head API, instead, and modify the usages to suit.
As a side-effect, the maximum number of synths has also become redundant.

Signed-off-by: Justin Skists <justin.skists@juzza.co.uk>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>