PM / devfreq: fix NULL pointer dereference for freq_table

Some devices freq_table is NULL, when show available_frequencies,
shouldn't access freq_table if max_state is 0.

Change-Id: I58ffaa23c1d27c0fe147c6d4f755244c3f69fbc0
Signed-off-by: Qiwei Liu <qiweil@codeaurora.org>