PCI/MSI: Switch msi_capability_init() to guard(msi_desc_lock)

Split the lock protected functionality of msi_capability_init() out into a
helper function and use guard(msi_desc_lock) to replace the lock/unlock
pair.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/all/20250319105506.504992208@linutronix.de