staging: comedi: adv_pci1710: fix ai (*insn_read)

An (*insn_read) can only happen if the subdevice is in a non-busy state,
i.e. an async command is not running. The board reset and subdevice
(*cancel) will ensure that the control bits (devpriv->ctrl) are already
cleared.

The (*insn_read) only needs to enable the software trigger before reading
samples. It should also disable the software trigger when done. Fix the
(*insn_read) to do this.

For aesthetics, rename the function so it has namespace associated with
the driver.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>