staging: comedi: das16m1: minor cleanup to das16m1_ai_insn_read()

The (*insn_read) functions return the number of data values read. The 'n'
value is correct but for clarity change the return to 'insn->n'.

For aesthetics, change the 'n' loop variable name to 'i'. That's more common
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>