staging: comedi: comedi_test: rename waveform_ai_interrupt()

`waveform_ai_interrupt()` is a timer expiry function used to generate
fake waveform data for an analog input subdevice.  Rename it to
`waveform_ai_timer()`.

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>