power: Use macros instead of string literals.

Replace multiple instances of string literals with
macros so as to minimize the risk of typos.

	* Also fix up loggin for ondemand sysfs_write (ALOGW->ALOGV)

Signed-off-by: bmc08gt <bmc08gt.teamb00sted@gmail.com>
Change-Id: I852ad370c2e371bb66e754ecc92e60f38c501047