drivers: cpuidle: lpm-levels: Return non-zero error in .select

cpuidle select calls expects an index into the cpuidle states array as
the return value. The return value is used to index into the state array
and therefore cannot be a negative value or an error return. Further,
its a general understanding across all architectures that use
cpuidle to have a WFI (or equalivalent) mode always present and
never disabled.

To this effect, disallow WFI modes to be disabled and return index 0
(index of WFI state) as the default if no suitable mode is available.

Change-Id: I5a14b0c5549f37427f97ba609321c646e02caae4
Signed-off-by: Lina Iyer <ilina@codeaurora.org>