staging: comedi: rtd520: fix ai_read_n() async->cur_chan use

This functions uses the async->cur_chan to determine if the current channel
is using a bipolar range and the sample needs to be munged. The cur_chan is
never incremented so all the samples are munged based on the fist channel
in the cmd->chanlist.

Bump the cur_chan after writing each sample. This fixes the code so that
the munging will be done correctly.

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>