PM / devfreq: bimc-bwmon: Don't always round up BWMON5 measurements

All BWMON5 counters currently have very coarse resolution. So, rounding up
to the next resolvable measurement results in very high bandwidth
measurement event for use cases that generate almost no traffic. So,
instead of always rounding up to the next resolvable measurement, round up
only when the measurement is non-zero.

Change-Id: I347c9a9a3863291d2b94a4530d32faf2b1e996e0
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>