net: macb: simplify macb_adj_dma_desc_idx()

net: macb: simplify macb_adj_dma_desc_idx()

The function body uses a switch statement on bp->hw_dma_cap and handles
its four possible values: 0, is_64b, is_ptp, is_64b && is_ptp.

Instead, refactor by noticing that the return value is:
   desc_size * MULT
with MULT = 3 if is_64b && is_ptp,
            2 if is_64b || is_ptp,
            1 otherwise.

MULT can be expressed as:
   1 + is_64b + is_ptp

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://patch.msgid.link/20251014-macb-cleanup-v1-7-31cd266e22cd@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>