sc16is7xx: Use threaded IRQ

Use a threaded IRQ handler to get rid of the irq_work kthread.
This also allows for the driver to use interrupts generated by
a threaded controller.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Link: https://lore.kernel.org/r/20200521091152.404404-5-daniel@zonque.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>