net: dsa: mxl862xx: use RST_DATA to skip writing zero words

Issue the firmware's RST_DATA command before writing data payloads that
contain many zero words. RST_DATA zeroes both the firmware's internal
buffer and the MMD data registers in a single command, allowing the
driver to skip individual MDIO writes for zero-valued words. This
reduces bus traffic for the common case where API structs have many
unused or default-zero fields.

The optimization is applied when at least 5 zero words are found in the
payload, roughly the break-even point against the cost of the extra
RST_DATA command round-trip.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/d10bd6ad5df062d0da342c3e0d330550b3d2432b.1774185953.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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