spi/omap: Use a workqueue per omap2_mcspi controller

Currently all the spi controllers share the work queue.
This patch allocates a work queue per controller.

Signed-off-by: Steve Wilkins <steve.wilkins@raymarine.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>