asoc: sm6150: Add proper checks for ch_num

Check the value of ch_num and return if
less than zero to avoid out of bound access
in cdc_dma_cfg arrays.

Change-Id: Ie59f5e7ed063d65a61033a142acd2bcf2287c39b
Signed-off-by: Tanya Dixit <tdixit@codeaurora.org>