staging: speakup: Simplify "NULL" comparisons

Fixed coding style for null comparisons in speakup driver to be more
consistant with the rest of the kernel coding style.
Replaced 'x != NULL' with 'x' and 'x = NULL' with '!x'.

Signed-off-by: Arushi Singhal <arushisinghal19971997@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>