toradex: tdx-cfg-block: fix switch to user partition

toradex: tdx-cfg-block: fix switch to user partition

The Toradex configuration block is stored in the first boot partition of
the eMMC. After reading the configuration block, U-Boot switches back to
the user partition. Currently, this operation always targets mmc device
0, even when the configuration block is stored on mmc device 2.

This patch addresses the issue by switching the mmc device set in
CONFIG_TDX_CFG_BLOCK_DEV to the user partition, rather than using the
hardcoded device 0.

Fixes: a2777ecb9d11 ("toradex: config block handling")
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>