Bluetooth: hidp: Let hidp_send_message return number of queued bytes

Let hidp_send_message return the number of successfully queued bytes
instead of an unconditional 0.

With the return value fixed to 0, other drivers relying on hidp, such as
hidraw, can not return meaningful values from their respective
implementations of write(). In particular, with the current behavior, a
hidraw device's write() will have different return values depending on
whether the device is connected via USB or Bluetooth, which makes it
harder to abstract away the transport layer.

Signed-off-by: Fabian Henneke <fabian.henneke@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>