dmaengine: iop-adma: remove set but not used variable 'slots_per_op'

dmaengine: iop-adma: remove set but not used variable 'slots_per_op'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/dma/iop-adma.c: In function iop_adma_tx_submit:
drivers/dma/iop-adma.c:367:6: warning:
 variable slots_per_op set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20190821121908.7468-1-yuehaibing@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>