dmaengine: pl330: Initialize pl330 for pl330_prep_dma_memcpy after NULL check of pch

Currently pch pointer is already dereferenced before NULL check
and thus we are getting below warning:
warn: variable dereferenced before check 'pch'

So initialize struct pl330_dmac *pl330 after NULL check
of dma_pl330_chan *pch.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Reviewed-by: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>