dmaengine: pch_dma: remove unused ‘cookie’

In pd_tx_submit(), variable ‘cookie’ is initialized but never used, which
leads to warning with W=1

drivers/dma/pch_dma.c: In function ‘pd_tx_submit’:
drivers/dma/pch_dma.c:420:15: warning: variable ‘cookie’ set but not used [-Wunused-but-set-variable]
  dma_cookie_t cookie;

So remove it.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>