staging: comedi: dmm32at: tidy up dmm32at_ai_rinsn()

For aesthetics, rename this function to dmm32at_ai_insn_read().

Get the 'chan' and 'range' from the chanspec when declaring the local
variables. Remove the unecessary masking of the 'chan'.

Remove some unnecessary comments.

Change the final return to 'insn->n' to clarify the return value.

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>