staging: comedi: adl_pci9118: move analog output (*insn_write) function

For aesthetics, move the analog output (*insn_write) function so it
is not located in the middle of the analog input functions. Also,
rename it for consistency.

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>