PM / devfreq: mtk-cci: Fix variable deferencing before NULL check

smatch warning:
drivers/devfreq/mtk-cci-devfreq.c:135 mtk_ccifreq_target()
warn: variable dereferenced before check 'drv' (see line 130)

This is based on static analysis only. Compilation tested.

Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>