wishbone: Let write callback talk to slave directly and process MSI.

After handle_write_cfg(, WBA_ERR, ) the function wakes a worker which
will talk to the hardware and process the MSI. This worker can be
delayed if the system is busy leading to latency spikes during the
communication.

The detour via the worker can be avoided and the request can be
completed within the write request. Doing it within handle_write_cfg()
could be possible because wishbone_process_msi() is using its own buffer
for communication and only the locking is in the way.
It looks safer and simpler to simply wait until all locks are released
and process the MSI afterwards if needed.
This also mimics the previous workflow except that there is less delay
now.

Don't schedule the worker from handle_write_cfg() but process the MSI
request directly from the write callback if needed.

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