iommu/ipmmu-vmsa: Convert to read_poll_timeout_atomic()

Use read_poll_timeout_atomic() instead of open-coding the same
operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/649c7e09841b998c5c8d7fc274884a85e4b5bfe9.1689599528.git.geert+renesas@glider.be


Signed-off-by: Joerg Roedel <jroedel@suse.de>