greybus: sdio: fix work-queue leak and use-after-free

A single global work-queue pointer was used for the per-connection
workqueue, something which would lead to memory leaks and all sorts of
bad things if there are ever more than one SDIO connection in a system.

Also add the missing error handling when allocating the queue.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>