soc: ti: ti_sci_inta_msi: Switch MSI descriptor locking to guard()

Convert the code to use the new guard(msi_descs_lock).

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/all/20250319105506.203802081@linutronix.de