pci/switchtec: Don't use completion's wait queue

The poll callback is using completion's wait_queue_head_t member and
puts it in poll_wait() so the poll() caller gets a wakeup after command
completed. This does not work on RT because we don't have a
wait_queue_head_t in our completion implementation. Nobody in tree does
like that in tree so this is the only driver that breaks.

Instead of using the completion here is waitqueue with a status flag as
suggested by Logan.

I don't have the HW so I have no idea if it works as expected, so please
test it.

Cc: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>