staging: comedi: usbduxfast: Improve unlocking of a mutex in usbduxfast_ai_insn_read()

[SF Markus Elfring]

> FYI, you are responding to someone who is on my blacklist

I am curious if this communication setting will ever be adjusted.


> and I never accept patches from.

The history shows that our collaboration style changed over time.
I got a few update suggestions integrated (also by you) because
other contributors found them good enough to repeat them.

Regards,
Markus