gpio: mpsse: ensure worker is torn down

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.

Signed-off-by: Mary Strodl <mstrodl@csh.rit.edu>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20251014133530.3592716-3-mstrodl@csh.rit.edu
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>