BAM_DESC_CNT_TRSHLD register is global register, which can only be written
when BAM is in master mode, So check the mode of operation before writing
it.

Without this check SOC's xPU would catch such access and crash the system.
First noticed on DB820c while testing SLIMBus BAM.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>