greybus: use little-endian in PWM requests

The PWM config request defines two 32-bit values using u32. All
over-the-wire values have to be in little-endian format.  Fix this.

Signed-off-by: Alex Elder <elder@linaro.org>
Acked-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>