spi: spi-ti-qspi: Remove unused dma_dev variable

commit 1351aaeb ("spi: spi-ti-qspi: Use dma_engine wrapper for dma
memcpy call") introduced this warning:

drivers/spi/spi-ti-qspi.c: In function 'ti_qspi_dma_xfer':
drivers/spi/spi-ti-qspi.c:398:21: warning: unused variable 'dma_dev' [-Wunused-variable]
  struct dma_device *dma_dev = chan->device;

Fix it by removing the unused variable.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@kernel.org>