thermal: gov_power_allocator: Remove excessive local variables

Local variable 'ret' in allocate_power() is only used in the return
statement, so drop it.

Local variable 'trip_max' in allocate_power() is only used for caching
the params->trip_max value which may as well be accessed directly as
needed, so drop it either.

This change is not expected to alter the general functionality.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>