drivers: lpm: cleanup error handling

Clean up error handling in power drivers. Also use the recommended
variants of kzalloc, snprintf etc. Remove unwanted debug prints and warn
and refactor redundant code as functions.

Change-Id: If41d285e45a81cab9bd3458f2131dcacd2950492
Signed-off-by: Lina Iyer <ilina@codeaurora.org>