net: macb: introduce DMA descriptor helpers (is 64bit? is PTP?)

net: macb: introduce DMA descriptor helpers (is 64bit? is PTP?)

Introduce macb_dma64() and macb_dma_ptp() helper functions.
Many codepaths are made simpler by dropping conditional compilation.

This implies two additional changes:
 - Always compile related structure definitions inside <macb.h>.
 - MACB_EXT_DESC can be dropped as it is useless now.

The common case:

	#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
		struct macb_dma_desc_64 *desc_64;
		if (bp->hw_dma_cap & HW_DMA_CAP_64B) {
			desc_64 = macb_64b_desc(bp, desc);
			// ...
		}
	#endif

Is replaced by:

	if (macb_dma64(bp)) {
		struct macb_dma_desc_64 *desc_64 = macb_64b_desc(bp, desc);
		// ...
	}

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20251014-macb-cleanup-v1-9-31cd266e22cd@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>