devfreq: allow zero values in opp table

commit ab8f58ad ("PM / devfreq: Set
min/max_freq when adding the devfreq device") introduces a change
where an error in finding the ceil or floor is returned as a
zero frequency. This return of zero is treated as error condition
from the callsites.

It is perfectly valid for a device to have a zero frequency, usually
it denotes a power collapse. This valid return of zero frequency ends
up being treated as an error condition.

Fix it.

Change-Id: I5eb6fa622d6fe09207746b96dce05d0b58b233dc
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>