staging: comedi: dt2814: Call dt2814_ai_clear() during initialization

The Comedi "attach" handler `dt2814_attach()` writes to the Control
register to turn off the timer enable 'ENB' bit, which triggers a
conversion.  It then sleeps awhile and checks the Status register,
returning an error if the ERR bit is set.  However, the ERR bit could
have been set due to the conversion being triggered while the A/D
converter was busy (unlikely) or due to the conversion being triggered
before some previous sample had been read from the A/D Data register.

Replace the existing code with a call to `dt2814_ai_clear()` which waits
for any conversion to finish and then clears any unread data or error
condition.  A non-zero return value from `dt2814_ai_clear()` indicates a
time-out while waiting for the A/D converter to become non-busy.  Return
an error in that case.

Signed-off-by: Ian Abbott <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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