staging: comedi: dt9812: cleanup analog in subdevice (*insn_read)

For aesthetic reasons, rename the function to help with grepping and
rename some of the local vars.

dt9812_analog_in() can fail. Make sure to check for any failure and
return the errno.

The comedi core expects the (*insn_read) functions to return either
an errno or the number of samples read. Change the final return to
insn->n to make this clearer.

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>