printk: fix failure to return error in devkmsg_poll()

Error value got overwritten instantly.

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>