staging: speakup: kobjects: Delete comparison

Comparison to NULL is not necessary because 'if' statement tests if
'synth' is NULL anyway.

Delete comparison in 'if' structure to fix the checkpath.pl issue :
CHECK: Comparison to NULL could be written 'synth'.

Signed-off-by: Tamara Diaconita <diaconita.tamara@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>