Staging: speakup: replacing simple_strtoul with kstrtoul

Fix checkpatch warning: WARNING: simple_strtoul is obsolete, use
kstrtoul instead

Signed-off-by: Ashvini Varatharaj <ashvinivaratharaj@gmail.com>
Reviewed-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>