net: dsa: vsc73xx: speed up MDIO bus to max allowed value

According to the datasheet, the VSC73xx family's maximum internal MDIO bus
speed is 20 MHz. It also allows disabling the preamble.

This commit sets the MDIO clock prescaler to the minimum value and
disables the preamble to speed up MDIO operations.

It doesn't affect the external bus because it's configured in a different
subblock.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20240731203455.580262-1-paweldembicki@gmail.com


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