net80211: clean up time comparison in HT code

[adrian]

Summary

There's at least one place where the time comparison is wrong, leading
to some bad beahaviour around deciding to try and establish/reestablish
AMPDU TX sessions.

Start addressing these!

[bz]

I don't get the correlation between your commit message and these changes. Is there an actual change (due to casting) in this change? Or is there a problem "somewhere" but this change does not necessarily address that?

[adrian]

yeah, without it the ampdu retry stuff eventually stops retrying until the counter loops around again for the comparison to work