dmaengine: pl330: Fix overflow when reporting residue in memcpy

During memcpy operations the residue was always set to an u32 overflowed
value.

In pl330_tx_status() function number of currently transferred bytes was
subtracted from internal "bytes_requested" field. However this
"bytes_requested" was not initialized at start to length of memcpy
buffer so transferred bytes were subtracted from 0 causing overflow.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Fixes: aee4d1fa ("dmaengine: pl330: improve pl330_tx_status() function")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>