dmaengine: cppi: remove unused and bogus check

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: David Binderman <linuxdev.baldrick@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>