toradex: common: Remove #ifdef usage for 2nd ethaddr

Fix checkpatch warn, use `IS_ENABLED(CONFIG_TDX_CFG_BLOCK_2ND_ETHADDR)`
instead of `#ifdef CONFIG_TDX_CFG_BLOCK_2ND_ETHADDR`.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>