iwlwifi: mvm: clean up invalid station handling

Using IWL_MVM_STATION_COUNT and IWL_INVALID_STATION together
isn't a good idea as they have different values. Always use
IWL_MVM_STATION_COUNT for an invalid station in MVM and move
the definition of the IWL_INVALID_STATION constant into the
DVM driver to avoid making such mistakes again. The one use
in the transport code can be hard-coded to -1 instead as the
station ID is passed as an integer there.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>