powerpc/powernv/ioda2: Reduce upper limit for DMA window size

We use PHB in mode1 which uses bit 59 to select a correct DMA window.
However there is mode2 which uses bits 59:55 and allows up to 32 DMA
windows per a PE.

Even though documentation does not clearly specify that, it seems that
the actual hardware does not support bits 59:55 even in mode1, in other
words we can create a window as big as 1<<58 but DMA simply won't work.

This reduces the upper limit from 59 to 55 bits to let the userspace know
about the hardware limits.

Fixes: 7aafac11 "powerpc/powernv/ioda2: Gracefully fail if too many TCE levels requested"
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/pci-ioda.c

@@ -2840,7 +2840,7 @@ static long pnv_pci_ioda2_table_alloc_pages(int nid, __u64 bus_offset,
 	level_shift = entries_shift + 3;
 	level_shift = max_t(unsigned, level_shift, PAGE_SHIFT);
 
-	if ((level_shift - 3) * levels + page_shift >= 60)
+	if ((level_shift - 3) * levels + page_shift >= 55)
 		return -EINVAL;
 
 	/* Allocate TCE table */