spi/omap2_mcspi: Use transaction speed if provided

omap2_mcspi_transfer() gets called in omap2_mcspi_work() when the
transaction speed_hz or bits_per_word fields are non-zero.

omap2_mcspi_transfer() does not look at the speed_hz field so
the override speed value is ignored.

The code should probably change to one of these options.

1. Skip the call to omap2_mcsp_transfer() if the only reason was a
non-zero speed_hz and it's not going to be used.

2. Use the new speed_hz value provided

The patch below uses the speed_hz value.

Signed-off-by: Scott Ellis <scott@jumpnowtek.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>