iwlwifi: mvm: add debugfs file for fixed reduced tx power

Allow fixing the tx power reduction through debugfs
file.

The change doesn't take affect immediately, but
will be considered the next time tpc is evaluated.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>