spi/omap2: fix uninitialized variable

fixes below compilation warning.  The variable doesn't actual ever get
used uninitialized, but that's no reason to be sloppy.

drivers/spi/omap2_mcspi.c: In function 'omap2_mcspi_txrx_dma':
drivers/spi/omap2_mcspi.c:301: warning: 'elements' may be used uninitialized in this function

Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
[grant.likely: amended description]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>