libertas: use private SDIO workqueue to avoid scheduling latency

The libertas SDIO interface scheduled the packet worker, resulting in
unwanted latency for every data packet or command sent to the firmware.
Fix a bug on the SDIO probe error path too.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>