sched: walt: Fix thermal un-mitigation bug

do_thermal_cap attempted to optimize away unnecessary division
when multiple updates came in with 'thermal_max_freq' set to
the maximum possible frequency. However this introduced a bug
that prevented resetting capacity to the max possible value when
thermal finally allowed the maximum frequency to be set. Fix it.

Change-Id: I49b6ca55cf8226358678c6d313e130b15dc2242d
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>