iommu/amd: Fix section mismatch warning for detect_ivrs()

A recent commit introduced this section mismatch warning:

	WARNING: modpost: vmlinux.o(.text.unlikely+0x22a1f): Section mismatch in reference from the function detect_ivrs() to the variable .init.data:amd_iommu_force_enable

The reason is that detect_ivrs() is not marked __init while it should
be, because it is only called from another __init function. Mark
detect_ivrs() __init to get rid of the warning.

Fixes: b1e650db ("iommu/amd: Add amd_iommu=force_enable option")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20210608122843.8413-1-joro@8bytes.org