genirq/msi: Rename msi_[un]lock_descs()

Now that all abuse is gone and the legit users are converted to
guard(msi_descs_lock), rename the lock functions and document them as
internal.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huwei.com>
Link: https://lore.kernel.org/all/20250319105506.864699741@linutronix.de