ia64: simplify code flow around swiotlb init

Before the change CONFIG_INTEL_IOMMU && !CONFIG_SWIOTLB && !CONFIG_FLATMEM
could skip `set_max_mapnr(max_low_pfn);` if iommu is not present on
system.

Link: https://lkml.kernel.org/r/20210328202439.403601-1-slyfox@gentoo.org


Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>