iwlwifi: mvm: simplify TX power setting

There's no need to double this code, just put it into the common
code that's called in all the cases.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210117164916.1f75d426ebe4.I58f6612f7e168c655bdef206a53e5bc117c84cf5@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>