In cppi41_dma_prep_slave_sg() variable num is initialized to zero, but never
updated and a BUG_ON is checked for it being greater than zero which will be
always false.

Remove the bogus check and this variable
Reported-by: NDavid Binderman <linuxdev.baldrick@gmail.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>