irda: Fix netlink error path return value

Fix an incorrect return value check of genlmsg_put() in irda_nl_get_mode().
genlmsg_put() does not use ERR_PTR() to encode return values, it just
returns NULL on error.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>