Bluetooth: HFP_HF: Avoid at status overwrote by sending

There is an issue that the sending of AT+command will overwrite the at
status when the at is handling receiving.

Add a flag `BT_HFP_HF_FLAG_RX_ONGOING` to flag the receiving is
working.

When the flag is set, suspend the TX sending.

Add a worker to restart the tx sending.

After the flag `BT_HFP_HF_FLAG_RX_ONGOING` is cleared, submit the TX
worker to restart the tx sending.

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