staging: comedi: dt9812: cleanup analog out subdevice (*insn_write)

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

The dt9812_analog_out() function can fail. Make sure to check for any
failure and return the errno.

The comedi core expects the (*insn_write) functions to return either
an errno or the number of samples written. 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>