staging: comedi: cb_pcidas: cleanup dac08_write()

Reverse the logic of the test against the cached value so that
the function only has one exit point.

Make the logic of the dac write a bit clearer by create a local
variable for the address used in the outw() calls and masking the
value to write and setting the enable bits before doing the outw()
calls. Add a comment just to make sure it's clear.

Also, add a comment about the 'return 1;'.  This should be
insn->n which is the number of data parameters used to do the
dac08_write_insn() but the insn is not a parameter to this
function.

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>