Bluetooth: HFP_AG: Change "-EOPNOTSUPP" to "-ENOEXEC"

There is an error reported in platform `native_sim` that the value of
`EOPNOTSUPP` is same as `ENOTSUP`.
It cause the building error `duplicate case value '-95'` reported.

Use `ENOEXEC` to replace `EOPNOTSUPP` to fix the building issue.

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