perf: Cache service name for fan target sensors

Cache the service name for each fan target sensor path and use that
service when a new target speed is set on the Target property.

Tested:
    Fan target sensor created and set speed functions the same

Change-Id: I3e25e355cf5d31ce814a73c801c6f086fa45531a
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
2 files changed