staging: comedi: pcmad: tidy up pcmad_ai_insn_read()

Use a local variable to read and munge the analog input data instead
of directly using the 'data' pointer passed to the function.

(*insn_read) functions either return an errno or the number of data
values read. Change the final return to insn->n to make this clearer.

Tidy up the function.

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