checkpatch.pl: fix CAST detection

We should only claim that something is a cast if we did not encouter a
token before, that did set av_pending.

This fixes the operator * in the line below to be detected as binary (vs
unary).

kmalloc(sizeof(struct alphatrack_ocmd) * true_size, GFP_KERNEL);

Reported-by: Audun Hoem <audun.hoem@gmail.com>
Signed-off-by: Florian Mickler <florian@mickler.org>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>