staging: comedi: comedi_test: remove nano_per_micro

The `static const int nano_per_micro` variable is set to 1000, the
number of nanoseconds in a microsecond.  Remove it and use the
`NSEC_PER_USEC` macro from <linux/time.h> instead.

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>