audio_hw: make mixer_path obey limits of MIXER_PATH_MAX_LENGTH

Test: compile and sanity
Bug: 117999014
Change-Id: Ic4be8725765b83a2d5bc00414d663e3884be4ab2
(cherry picked from commit 11f10e295e6cdce6ac94da428d747673bdca6a59)
1 file changed