iommu/ipmmu-vmsa: Make IPMMU_CTX_MAX unsigned

Make the IPMMU_CTX_MAX constant unsigned, to match the type of
ipmmu_features.number_of_contexts.

This allows to use plain min() instead of type-casting min_t().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>