drivers: cpuidle: lpm-levels: Fix KW issues with idle state idx < 0

Idle state calculcations will need to return the state chosen as an
integer. The state chosen is used as a index into the array and as such
cannot be negative value. Do not return negative errors from the
calculations. By default, the state returned wil be zero.

Change-Id: Idb18e933f385cf868fe99fa6a2783f6b8e84c196
Signed-off-by: Lina Iyer <ilina@codeaurora.org>