dmaengine: qcom: bam_dma: fix some doc warnings.

Building kernel with W=1 throws up below warnings:
bam_dma.c:459: warning: Function parameter or member 'dir'
	not described in 'bam_chan_init_hw'
bam_dma.c:697: warning: Function parameter or member 'chan'
	not described in 'bam_dma_terminate_all'
bam_dma.c:697: warning: Excess function parameter 'bchan'
	description in 'bam_dma_terminate_all'
bam_dma.c:964: warning: Function parameter or member 'bchan'
	not described in 'bam_start_dma'

Fix these!.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>