staging: speakup: add spaces around arithmetic operators

Add space around arithmetic operators ('+', '-' and '*') to conform to
Linux kernel coding style. Problem found using checkpatch.

Signed-off-by: Santha Meena Ramamoorthy <santhameena13@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>