iommu/mediatek: Mark static functions in headers inline

This was an oversight while merging these functions. Fix it.

Cc: Honghui Zhang <honghui.zhang@mediatek.com>
Fixes: 9ca340c9 ('iommu/mediatek: move the common struct into header file')
Signed-off-by: Joerg Roedel <jroedel@suse.de>