sparc/iommu: fix typo s/265KB/256KB/

IOMMU_NPTES is 64K PTEs, so the size is 256KB (= 64K * sizeof(iopte_t))

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>