STPK-1429 Take into account poll-mode sensors in sampling rate code

STPK-1429 Take into account poll-mode sensors in sampling rate code

Poll-mode sensors firing rate is entirely controlled by software,
and more precisely by our own poll loop, so be careful when
handling the desired sampling rate for these. On the T100, we
have both poll mode sensors (such as the AK magnetometer) and
regular character-device sensors (accel, gyro) on iio device 0,
so also check for poll-mode devices when arbitrating the device
level sampling rate.

Issue: STPK-1429

Change-Id: I561d25b5afa83fd0360b9197b1dd8b6fd7dffca5
Signed-off-by: Patrick Porlan <patrick.porlan@intel.com>