PM / devfreq: Fix NULL pointer dereference if freq_table is empty

If max_state is 0, freq_table will be empty. Change do-while loop to
while loop to avoid dereferencing freq_table.

Change-Id: I4a24e9b8cab8073db429c74e627b7fb50076ea93
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>