iwlwifi: mvm: add support for negative temperatures

The driver should support also negative temperatures.
So there is a need to separate between the return value and
temperature in order to be able to distinguish between
a negative temperature and error value.

Signed-off-by: Chaya Rachel Ivgi <chaya.rachel.ivgi@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>