spi: davinci: remove set but not used variable 'pdata'

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

drivers/spi/spi-davinci.c: In function 'davinci_spi_setup':
drivers/spi/spi-davinci.c:422:36: warning:
 variable 'pdata' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>