ena: Fix crash when interrupt arrives after admin command timeout

[osamaabb_amazon.com]

Summary

Bug description:
When invoking an admin command, in interrupt mode, if the interrupt
is received after timeout and also after the calling function finished
running, the response will be written into a memory that is no longer
valid.

We fix this by applying the following:

1. Verifying that comp_ctx is occupied during the interrupt handler, otherwise it doesn't make sense to update its fields.
2. Set user_cqe to NULL when the admin command times out. When the interrupt handler is later invoked, it will check if it is NULL and will not use it.

Approved by: cperciva
MFC after: 2 weeks
Sponsored by: Amazon, Inc.