spi: davinci: use spi->cs_gpio directly

Use spi->cs_gpio directly to remove the following build warning:
drivers/spi/spi-davinci.c:219:6: warning: 'gpio' may be used uninitialized in this function

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@kernel.org>