pcie-wb: Use threaded interrupts and avoid the kworker.

The interrupt service routine schedules a worker in order to perform the
main task including signaling userspace. This can lead to increased
latency since the worke is not sychronized with the interrupt handler
and be invoked "later" depending on the load on the system.

This can be avoided by using a threaded interrupt and invoking the work
directly in the threaded interrupt. There are still other places which
invoke wishbone_slave_ready() (and use the worker) in order to judge if
this is sane to do or not.

Use threaded interrupts and perform the work in the threaded interrupt.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>