dmaengine: pl330: fix return status on pending transfers

The pl330_tx_status() function returns the desc->status if the
dma_cookie_status() call does indicate the cookie completed,
however the desc->status is not look directly compatible. Sparse
throws the following warning:

pl330.c:2262:35: warning: mixing different enum types
pl330.c:2262:35:     int enum desc_status  versus
pl330.c:2262:35:     int enum dma_status

Attempt to fix this by adding a switch statement to turn the
desc->status into a dma_status.

Note, this has only been tested with the dmatest suite.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
--
Vinod Koul <vinod.koul@intel.com>
Dan Williams <dan.j.williams@intel.com>
DMA List <dmaengine@vger.kernel.org>
Maxime Ripard <maxime.ripard@free-electrons.com>
Jassi Brar <jassisinghbrar@gmail.com>
Liviu Dudau <Liviu.Dudau@arm.com>
Linux ARM Kernel <linux-arm-kernel@lists.infradead.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>