iwlwifi: yoyo: don't allow changing the domain via debugfs

We don't want to allow changing the domain via debugfs so that we can
apply the domain to all TLV types more easily (doing some at runtime
is difficult due to buffer allocations etc.).  Change the
fw_dbg_domain debugfs file to be read-only and remove the write
function.

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