Fix audio port "mix" role parameter conversion

'usecase.source' field is used for AUDIO_PORT_ROLE_SINK,
'usecase.stream' field is used for AUDIO_PORT_ROLE_SOURCE.

The HIDL<->legacy conversion code was doing it opposite.

Test: make
Change-Id: I93722f422a8f226f11a6be97215453af3234029c
1 file changed