staging: vt6655: replace NULL comparison with variable

Replace NULL comparison of the variable with just the variable name to
conform to the Linux kernel coding style. Issue found using checkpatch.

Signed-off-by: Santha Meena Ramamoorthy <santhameena13@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>