staging: comedi: das08: rename das08_ai_rinsn()

`das08_ai_rinsn()` is the handler for Comedi `INSN_READ` instructions
for the AI subdevice.  Rename the function to `das08_ai_insn_read()` for
consistency.

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