dmaengine: qcom: bam_dma: fix invalid assignment warning

Building kernel with W=1 throws below invalid assignment warnings.
bam_dma.c:676:44: warning: invalid assignment: +=
bam_dma.c:676:44:    left side has type unsigned long
bam_dma.c:676:44:    right side has type restricted __le16
bam_dma.c:921:41: warning: invalid assignment: +=
bam_dma.c:921:41:    left side has type unsigned long
bam_dma.c:921:41:    right side has type restricted __le16

Fix them!.

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