[bug 667] Fix the notorious "No space for Tx" bug.

We send SYSTEM_CONFIG command after the TGI_KEY command if hardware
encryption is enabled. It sometimes causes a firmware stall (firmware
doesn't respond to any request) and finally bungs up the Tx send queue.
The solution is to send SYSTEM_CONFIG command in the post association
stage from a workqueue.

Signed-off-by: James Ketrenos <jketreno@linux.intel.com>