scmi_perf: Fix level within limits

When performance is evaluated and limits ar enot changed, level still
needs to be brought within the limits' range.
Currently it works only for the maximum limit, i.e. level is reduced if
max limit is pushed down.

Fix the level within both limit boundaries.

This is done only for the case where
BUILD_HAS_SCMI_PERF_PLUGIN_HANDLER is set.

Change-Id: I03d62dae42ad9907aabb90338b99b0a2bfe84c44
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>