spi: omap2-mcspi: Fix the redifine warning

Fix the below warning
drivers/spi/spi-omap2-mcspi.c:336:34: warning: symbol 'tx' shadows an earlier one
drivers/spi/spi-omap2-mcspi.c:327:12: originally declared here

So delete the u8 tx as it is assigned and not used(resigned afterwards).

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>