dmaengine: sf-pdma: remove unused 'desc'

'desc' variable is now defined but not used in sf_pdma_donebh_tasklet(),
causing this warning:

drivers/dma/sf-pdma/sf-pdma.c: In function 'sf_pdma_donebh_tasklet':
drivers/dma/sf-pdma/sf-pdma.c:287:23: warning: unused variable 'desc' [-Wunused-variable]

Remove this unused variable

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>