system/audio.h: Make function inline

This patch makes the audio_device_address_to_parameter inline. All of the other
helpers defined in this header are inline functions except for this one and this
causes compile failures when -Werror=unused-function is used.

BUG=21339022

Change-Id: I7db58bf7bed3b89b6752ec2cd2c1f358ba2a4d89