pm / devfreq: Fix uninitalized variable warning

drivers/devfreq/devfreq_qcom_fw.c: In function 'devfreq_qcom_fw_driver_probe':
drivers/devfreq/devfreq_qcom_fw.c:131:26: warning: 'prev_freq' may be used uninitialized in this function [-Wmaybe-uninitialized]

Fixes: 3c08d9b8 ("PM / devfreq: Add support for QCOM devfreq firmware")
Change-Id: Ic0dedbad700d71698cca311d5130b2ad1dd1412e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>