staging: kpc2000: use sizeof(var) in kzalloc call

Fixes checkpatch.pl warning "Prefer kzalloc(sizeof(*pcard)...) over
kzalloc(sizeof(struct kp2000_device)...)".

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