staging: comedi: mf6x4: Fix AI end-of-conversion detection

I have had reports from two different people that attempts to read the
analog input channels of the MF624 board fail with an `ETIMEDOUT` error.

After triggering the conversion, the code calls `comedi_timeout()` with
`mf6x4_ai_eoc()` as the callback function to check if the conversion is
complete.  The callback returns 0 if complete or `-EBUSY` if not yet
complete.  `comedi_timeout()` returns `-ETIMEDOUT` if it has not
completed within a timeout period which is propagated as an error to the
user application.

The existing code considers the conversion to be complete when the EOLC
bit is high.  However, according to the user manuals for the MF624 and
MF634 boards, this test is incorrect because EOLC is an active low
signal that goes high when the conversion is triggered, and goes low
when the conversion is complete.  Fix the problem by inverting the test
of the EOLC bit state.

Fixes: 04b56502

 ("comedi: Humusoft MF634 and MF624 DAQ cards driver")
Cc: <stable@vger.kernel.org> # v4.4+
Cc: Rostislav Lisovy <lisovy@gmail.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20201207145806.4046-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>