staging: comedi: comedi_test: use unsigned short for loopback values

The last sample values written to the AO subdevice channels by its
"insn_write" handler `waveform_ao_insn_write()` are stored in the member
array `ao_loopbacks[]` in the device private data `struct
waveform_private`.  They can be read back via the "insn_read" handler of
the AI subdevice `waveform_ai_insn_read()`.  As the stored sample values
are only 16 bits wide, change the type of the `ao_loopbacks[]` member to
`unsigned short` to save some space.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>