motion: Set interrupt interval properly for sensor in force mode
motion: Set interrupt interval properly for sensor in force mode

cl/301134 has a bug. If the AP wants a forced sensor (i.e. light) at
100Hz but a sampling frequency at 1s, we would still wake it up every
.1s instead of 1s.
Take in account force mode only when calculating the sampling frequency
not the interrupt interval.

BRANCH=smaug
BUG=b:25425420
TEST=Check the device goes to suspend even with 40Hz light sampling
rate:
echo 0 > /sys/bus/iio/devices/iio:device0/frequency
echo 40000 > /sys/bus/iio/devices/iio:device3/frequency
echo mem >/sys/power/state
Before it would resume just after suspend/while suspending.

Change-Id: Ie4fe36268cb1b04bc8f01ec885af84fad9e8b282
Signed-off-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/314315
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>

coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics