iwlwifi: move iwl_set_soc_latency to iwl-drv to be used by other op_modes

All the op_mode need to send this command as well. Instead of
duplicating the code from mvm, put the code in a common place.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200424194456.7f30f977f9bf.I060b51d0d66d09b9d1ee512e7de8f2d695a52152@changeid