PCI: vmd: Convert to lock guards

Convert lock/unlock pairs to lock guard and tidy up the code.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: rebase on dev_fwnode() conversion]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/836cca37449c70922a2bea1fb13f37940a7a7132.1750858083.git.namcao@linutronix.de