PM / devfreq: passive: Fix get_target_freq when not using required-opp

PM / devfreq: passive: Fix get_target_freq when not using required-opp

The 86ad9a24f21e ("PM / devfreq: Add required OPPs support to passive governor")
supported the required-opp property for using devfreq passive governor.
But, 86ad9a24f21e has caused the problem on use-case when required-opp
is not used such as exynos-bus.c devfreq driver. So that fix the
get_target_freq of passive governor for supporting the case of when
required-opp is not used.

Fixes: 86ad9a24f21e ("PM / devfreq: Add required OPPs support to passive governor")
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>