rtphdrext-clientaudiolevel: Fix level value being written by the extension

When level value is greater than 127, it was being clamped but this clamped value was not the one being actually used. For level values greater than 127 this resulted in an incorrect value being used. As an example, a level value of 187, after and'ed with 0x7F, it would result in 0x3B being reported as the level value.