Bluetooth: LE: CONN: Added bt_conn_le_set_default_phy function.

Added bt_le_set_default_phy in hci. This handles
the HCI_LE_Set_Default_PHY HCI command.
Added call to bt_conn_le_set_default_phy in central_gatt_write and
peripheral_gatt_write samples.

Signed-off-by: Ravinder Singh <ravinder.singh2@infineon.com>