Bluetooth: HFP_HF: Send AT+VGM and AT+VGS after SLC established

Send AT command AT+VGM and AT+VGS after SLC established.

The `VGM` and `VGS` can be set by calling function `bt_hfp_hf_vgs` and
`bt_hfp_hf_vgs` in HF connection callback.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>