staging: comedi: comedi_test: move modulo operations for waveform

The fake waveform generator functions, `fake_sawtooth()` and
`fake_squarewave()`, called from `fake_waveform()`, have a
`current_time` parameter which is the time since the start of a waveform
period.  The parameter value may be greater than the waveform period so
they do a modulo operation to bring it into range.  Do the modulo
operations outside the functions in `waveform_ai_interrupt()` so that
the waveform generator functions always get a `current_time` parameter
less than the waveform period and do not have to do the modulo operation
themselves.  Also, only do the modulo operations when the time since the
start of a waveform exceeds the waveform period.  Usually, several
samples are produced in each waveform period and modulo operations are
typically more expensive than a simple comparison.

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>