Update misleading comment in cifs_chan_update_iface

Since commit 8da33fd1 ("cifs: avoid deadlocks while updating iface")
cifs_chan_update_iface now takes the chan_lock itself, so update the
comment accordingly.

Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>