devfreq: update sample_ms tunnable store function

The sample_ms should never be larger than the devfreq polling
interval. If userspace tries to configure this invalid
configuration, return an error.

Devfreq polling interval should be always greater then the
sample_ms add check in the governor interval event to update
the polling interval accordingly.

Change-Id: I2d07c95668a7fc8e3b2f16e49448ad2e3da8785c
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>