gpio: mpsse: ensure worker is torn down

In the Linux kernel, the following vulnerability has been resolved:

gpio: mpsse: ensure worker is torn down

When an IRQ worker is running, unplugging the device would cause a

crash. The sealevel hardware this driver was written for was not

hotpluggable, so I never realized it.

This change uses a spinlock to protect a list of workers, which

it tears down on disconnect.