staging: speakup: Comparison to NULL.

Fix coding style issues, detected by checkpatch.pl "CHECK: Comparison to
NULL could be written !tty->ops->write"

Signed-off-by: Bhagyashri Dighole <digholebhagyashri@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>