Bluetooth: Mesh: Fix send input_complete before public key

According Mesh Profile Spec 5.4.2.4 Authentication, if device
use Input OOB Authentication method, should send input complete
pub after local public key has been acked.

`bt_mesh_input_string` or `bt_mesh_input_number` directly send
`input_complete`, however does not check whether the pub key has
been sent.

Mesh Provisioning timeout set to `60` seconds, so even this
probability is extremely low, it does not mean that there is no
such probability.

Signed-off-by: Lingao Meng <mengabc1086@gmail.com>