msm8994: audio: use correct length in msm_audio_ion_mmap

When compiling for msm8994 the dma_length field is set to 0,
making the use of the sg_dma_len() macro incorrect.

Instead of using this macro, we use the length field directly.

Change-Id: I75ce78243f02e1b2c540a03f7f44dbc5c106dba3
Acked-by: Hadar Sivan <hsivan@qti.qualcomm.com>
Signed-off-by: Lior Barenboim <liorb@codeaurora.org>