staging: typec: tcpm: Rewrite comparison to NULL pointer

Make code more concise and readable

Signed-off-by: Harsha Sharma <harshasharmaiitr@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>