iwlwifi: mvm: don't NUL-terminate low-latency debugfs file

There's obviously no reason to write a NUL-terminating byte into
the debugfs file. Apparently nobody tried to use this from a tool
that doesn't strip NUL bytes though, so we never noticed. Fix it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>