arm: mvebu: a38x: serdes: fix serdes config for USB3

arm: mvebu: a38x: serdes: fix serdes config for USB3

The electrical serdes configuration for USB3 expects an array as data
argument. For USB3 the second value is used (see data_arr_idx = USB3 =
1). However, because only one value is inside the array mv_seq_exec is
accessing an invalid element and the serdes is configured wrongly.

This wrong initialization is leading to an unreliable detection
mechanism for some USB3 devices. We were able to reproduce the issue
regularly with an LTE modem from Sierra Wireless (SM7455) where it was
not detected as USB3 device in 1/3 of all tests.

This commit fixes the issue by setting data_arr_idx to 0. This is the
same value as the original U-Boot from Marvell is using. There it is
called FIRST_CELL which is a define for 0.
See: https://github.com/MarvellEmbeddedProcessors/u-boot-marvell
commit 56f963ce4c ("fix: serdes: a38x, a39x: Fix USB3 serdes DB
initialization")

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Signed-off-by: René Straub <rene.straub@netmodule.com>
Reviewed-by: Stefan Roese <sr@denx.de>