staging: comedi: pcmda12: rename the analog output (*insn_{read, write}) functions

For aesthetic reasons. rename these functions so they have namespace
associated with the driver. Also remove the unnecessary '&' when setting
the callbacks.

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>