dma: pl330: fix a couple of compilation warnings

Move a couple of tests and do a minor refactor to avoid:

drivers/dma/pl330.c: In function 'pl330_probe':
drivers/dma/pl330.c:2929:215: warning: comparison of distinct pointer types lacks a cast [enabled by default]
drivers/dma/pl330.c: In function 'pl330_tasklet':
drivers/dma/pl330.c:2250:8: warning: 'pch' may be used uninitialized in this function [-Wuninitialized]
drivers/dma/pl330.c:2228:25: note: 'pch' was declared here
drivers/dma/pl330.c:2277:130: warning: 'pch' may be used uninitialized in this function [-Wuninitialized]
drivers/dma/pl330.c:2260:25: note: 'pch' was declared here

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>