cxgbe(4): Add functions to help synchronize "slow" operations (those not
on the fast data path) and use them instead of frobbing the adapter lock
and busy flag directly.

Other changes made while reworking all slow operations:

• Wait for the reply to a filter request (add/delete). This guarantees that the operation is complete by the time the ioctl returns.
• Tidy up the tid_info structure.
• Do not allow the tx queue size to be set to something that's not a power of 2.

MFC after: 1 week