ibmaem: don't query the entire sensor repository when reading energy meter

Currently, all sensors are read when the energy meter is queried via
sysfs.  This introduces a considerable amount of delay and variation in
the sysfs reading, which is not desirable when trying to profile energy
use.  Therefore, read only the energy meters when a sysfs query comes in
for them, and don't cache the results so that we always get the latest
reading.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>