spi: omap2-mcspi: Cleanup the omap2_mcspi_txrx_dma function

Currently in omap2_mcspi_txrx_dma the tx and the rx support is
interleaved. Make the rx related code in omap2_mcspi_rx_dma
and the tx related code omap2_mcspi_tx_dma and call the functions.

While at it remove the braces in the if statements which has only
one line.
Also fix ["foo * bar" to "foo *bar"] warn for the rx and tx variables.

Only a cleanup no functional change.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>