staging: comedi: adv_pci1710: fix AI INSN_READ for non-zero channel

Reading of analog input channels by the `INSN_READ` comedi instruction
is broken for all except channel 0.  `pci171x_ai_insn_read()` calls
`pci171x_ai_read_sample()` with the wrong value for the third parameter.
It is supposed to be the current index in a channel list (which is
always of length 1 in this case, so the index should be 0), but instead
it is passing the actual channel number.  `pci171x_ai_read_sample()`
checks the channel number encoded in the raw sample value read from the
hardware matches the channel number stored in the specified index of the
previously set up channel list and returns `-ENODATA` if it doesn't
match.  Since the index should always be 0 in this case, the match will
fail unless the channel number is also 0.  Fix it by passing 0 as the
channel index.

Note that when the bug first appeared, it was `pci171x_ai_dropout()`
that was called with the wrong parameter value.  `pci171x_ai_dropout()`
got replaced with `pci171x_ai_read_sample()` in commit 7fd2dae2
("staging: comedi: adv_pci1710: introduce pci171x_ai_read_sample()").

Fixes: 16c7eb60

 ("staging: comedi: adv_pci1710: always enable PCI171x_PARANOIDCHECK code")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org> # 3.16+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>