DaVinci: Improve DaVinci SPI speed.

I have updated this patch based on the comments [1] by Wolfgang Denk and
removed unused variables.
[1][http://lists.denx.de/pipermail/u-boot/2010-May/071728.html

]

Reduce the number of reads per byte transferred on the BUF register from 2 to 1 and
take advantage of the TX buffer in the SPI module. On LogicPD OMAP-L138 EVM,
SPI read throughput goes up from ~0.8Mbyte/s to ~1.3Mbyte/s. Tested with a 2Mbyte image file.
Remove unused variables in the spi_xfer() function.

Signed-off-by: Delio Brignoli <dbrignoli@audioscience.com>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>