Bluetooth: hidp: handle kernel_sendmsg() errors correctly

We shouldn't push back the skbs if kernel_sendmsg() fails. Instead, we
terminate the connection and drop the skb. Only on EAGAIN we push it back
and return.
l2cap doesn't return EAGAIN, yet, but this guarantees we're safe if it
will at some time in the future.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>