nfc: add missing attribute validation for deactivate target

Add missing attribute validation for NFC_ATTR_TARGET_INDEX
to the netlink policy.

Fixes: 4d63adfe ("NFC: Add NFC_CMD_DEACTIVATE_TARGET support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>