spi: omap2-mcspi: Reorder the wait_for_completion for tx

The commit  d7b4394e[Cleanup the omap2_mcspi_txrx_dma function]
changed the wait_for_completion order. Move the wait so that the
rx doesnot wait for the tx to complete.

Reported-and-tested-by: Sørensen, Stefan <Sorensen@polycom.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>