PM / devfreq: bw_hwmon: use unsigned parameter for bytes_to_mbps

In bytes_to_mbps function, the parameter is all unsigned,
so change the decleration of the function to include unsigned long
long to avoid compilation errors in 32 bit environment.
Also changed the return value as unsigned long to avoid
any data loss possible in 64 bit environment.

Change-Id: Ib3906bb4bc16086ae343764c5a718f6f005428c5
Signed-off-by: Maria Yu <aiquny@codeaurora.org>