staging: comedi: dt3000: rename dt3k_ai_insn()

For aesthetics, rename this subdevice (*insn_read) function to follow
the normal naming in comedi drivers.

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>