net: macb: single dma_alloc_coherent() for DMA descriptors

Move from 2*NUM_QUEUES dma_alloc_coherent() for DMA descriptor rings to
2 calls overall.

Issue is with how all queues share the same register for configuring the
upper 32-bits of Tx/Rx descriptor rings. Taking Tx, notice how TBQPH
does *not* depend on the queue index:

	#define GEM_TBQP(hw_q)		(0x0440 + ((hw_q) << 2))
	#define GEM_TBQPH(hw_q)		(0x04C8)

	queue_writel(queue, TBQP, lower_32_bits(queue->tx_ring_dma));
	#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
	if (bp->hw_dma_cap & HW_DMA_CAP_64B)
		queue_writel(queue, TBQPH, upper_32_bits(queue->tx_ring_dma));
	#endif

To maximise our chances of getting valid DMA addresses, we do a single
dma_alloc_coherent() across queues. This improves the odds because
alloc_pages() guarantees natural alignment. Other codepaths (IOMMU or
dev/arch dma_map_ops) don't give high enough guarantees
(even page-aligned isn't enough).

Two consideration:

 - dma_alloc_coherent() gives us page alignment. Here we remove this
   constraint meaning each queue's ring won't be page-aligned anymore.

 - This can save some tiny amounts of memory. Fewer allocations means
   (1) less overhead (constant cost per alloc) and (2) less wasted bytes
   due to alignment constraints.

   Example for (2): 4 queues, default ring size (512), 64-bit DMA
   descriptors, 16K pages:
    - Before: 8 allocs of 8K, each rounded to 16K => 64K wasted.
    - After:  2 allocs of 32K => 0K wasted.

Fixes: 02c958dd ("net/macb: add TX multiqueue support for gem")
Reviewed-by: Sean Anderson <sean.anderson@linux.dev>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Tested-by: Nicolas Ferre <nicolas.ferre@microchip.com> # on sam9x75
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250923-macb-fixes-v6-4-772d655cdeb6@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>