staging: kpc2000: simplify comparisons to NULL in core.c

Fixes checkpatch.pl warnings "Comparison to NULL could be written [...]"
and "Comparisons should place the constant on the right side of the
test".

Signed-off-by: Simon Sandström <simon@nikanor.nu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>