PM / devfreq: Fix out of bounds access of transition table array

The previous_freq value for a device could be an invalid frequency that
results in a error value being returned from devfreq_get_freq_level().
Check for an error value before using that to index into the transition
table.

Not doing this check will result in memory corruption when previous_freq is
not a valid frequency.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Change-Id: Ia9f9f3dabf14b26c4dff54fb2bac3e77f33458c0