iommu/omap: Use the correct type for SLAB_HWCACHE_ALIGN

The macro SLAB_HWCACHE_ALIGN is of type slab_flags_t, but is currently
assigned in the OMAP IOMMU driver using a unsigned long variable. This
generates a sparse warning around the type check. Fix this by defining
the variable flags using the correct type.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>