audio-hal: Check validity of 'usecase'

Usecase fetched from adev->usecase_list is NULL and hence
the audio HAL crashed while accessing 'usecase' field
Added NULL check before accessing 'usecase'.

Change-Id: I03e4dd673984ccbb192227d4750a6790be010e74
Signed-off-by: Srinivasa Reddy M N <quic_c_mullan@quicinc.com>