staging: comedi: usbdux: cleanup the private data 'outBuffer'

This buffer is used to cache the values that are written to the
analog output channels. Currently it only caches the single writes
to the channels using the (*insn_write) callback. The async command
writes are not cached. The buffer is also being kzalloc'ed during
the attach of the driver to a size much larger that required.

Rename the CamelCase buffer and change it to an array in the private
data of the correct size to cache the analog output channel values.

Modify the analog output urb callback so it updates the cached values
with those used for the asynchronous command to allow readback after
the command completes.

The sanity check of the index to dac_commands[] (i.e. the 'chan' being
written) is not needed. The chanlist_len will always be less than the
number of channels.

Also, fix the dev_err() message so it uses the proper device pointer.

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>