monitor: Move timer ops into TachSensor

Upcoming commits will touch the TachSensor timer
outside of the Fan class, and it will be cleaner to
encapsulate the operations in TachSensor.

Change-Id: I8584c44eb5dbe6bb93191a006d20dfc37596eeb3
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
2 files changed