drivers/nble: Add bt_conn_le_param_update implementation

This adds implementation of bt_conn_le_param_update function.
Connection parameters can be only issued while in slave role,
because, nRF51 supports this procedure over L2CAP only.
Closes ZEP-222.

Change-Id: I2bb5f718b60b8be279f333c9721629662c029a32
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>