staging: comedi: gsc_hpdi: usleep_range is preferred over udelay

Fix checkpatch issue: "CHECK: usleep_range is preferred over udelay; see
Documentation/timers/timers-howto.txt".  `udelay()` is only called once
from a place where sleeping is allowed.  Replace it with a call to
`usleep_range()` with a reasonable upper limit.

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>