net: phy: dp83822: fix transmit amplitude if CONFIG_OF_MDIO not defined

When CONFIG_OF_MDIO is not defined the index for selecting the transmit
amplitude voltage for 100BASE-TX is set to 0, but it should be -1, if there
is no need to modify the transmit amplitude voltage. Move initialization of
the index from dp83822_of_init to dp8382x_probe.

Fixes: 4f3735e8 ("net: phy: dp83822: Add support for changing the transmit amplitude voltage")
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: Dimitri Fedrau <dimitri.fedrau@liebherr.com>
Link: https://patch.msgid.link/20250317-dp83822-fix-transceiver-mdio-v2-1-fb09454099a4@liebherr.com


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