pch_dma: Reduce wasting memory

nr_channels is defined in "struct pch_dma".
and struct pch_dma_chan is defined in "struct pch_dma".
So, "sizeof(struct pch_dma_chan) * nr_channels" is unnecessary.

Signed-off-by: Tomoya MORINAGA <tomoya-linux@dsn.lapis-semi.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>