pci/switchtec: Replace completion wait queue usage for poll

The poll callback is using the completion wait queue and sticks it into
poll_wait() to wake up pollers after a command has completed.

This works to some extent, but cannot provide EPOLLEXCLUSIVE support
because the waker side uses complete_all() which unconditionally wakes up
all waiters. complete_all() is required because completions internally use
exclusive wait and complete() only wakes up one waiter by default.

This mixes conceptually different mechanisms and relies on internal
implementation details of completions, which in turn puts contraints on
changing the internal implementation of completions.

Replace it with a regular wait queue and store the state in struct
switchtec_user.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200321113240.936097534@linutronix.de