staging: comedi: dt2815: fix writing hi byte of analog output

commit ed87d33d

 upstream.

The DT2815 analog output command is 16 bits wide, consisting of the
12-bit sample value in bits 15 to 4, the channel number in bits 3 to 1,
and a voltage or current selector in bit 0.  Both bytes of the 16-bit
command need to be written in turn to a single 8-bit data register.
However, the driver currently only writes the low 8-bits.  It is broken
and appears to have always been broken.

Electronic copies of the DT2815 User's Manual seem impossible to find
online, but looking at the source code, a best guess for the sequence
the driver intended to use to write the analog output command is as
follows:

1. Wait for the status register to read 0x00.
2. Write the low byte of the command to the data register.
3. Wait for the status register to read 0x80.
4. Write the high byte of the command to the data register.

Step 4 is missing from the driver.  Add step 4 to (hopefully) fix the
driver.

Also add a "FIXME" comment about setting bit 0 of the low byte of the
command.  Supposedly, it is used to choose between voltage output and
current output, but the current driver always sets it to 1.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200406142015.126982-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>