Fix EpochsTFR.add_channels()

[cbrnr]

EpochsTFR.add_channels() is currently broken, since it adds the objects in the wrong dimension (0, but should be 1). I think this should be backported if possible.

[hoechenberger]

Thanks @cbrnr !

[drammock]

@cbrnr or @hoechenberger can you backport?

[cbrnr]

Sorry, I don't really have time to go through this process, but please see #12623.

[drammock]

@meeseeksdev backport to maint/1.7

[larsoner]

@drammock the next time we want to backport we should try using the backport-candidate label -- I added some text to the description field of the label that should make the bot auto-open a PR to backport the change once the PR is merged.

[larsoner]

(Though I see it was too late here since this was merged a while ago!)

[drammock]

(Though I see it was too late here since this was merged a while ago!)

...and already had that label :) though I guess I could have tried removing and re-adding the label, is that expected to work?

regardless: this was the backport candidate that spurred the discussion that led to trying the bot, so I wanted to get it open quickly before I forgot about it (again)