dynamic_debug: small cleanup in ddebug_proc_write()

This doesn't change behavior at all.  In the original code, if nwords was
zero then ddebug_parse_query() would return -EINVAL, now we just do it
earlier.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>