rockchip: rk3399: fix incorrect ifdef check on SPL_DM_REGULATOR

The check to perform is on CONFIG_SPL_DM_REGULATOR and not
SPL_DM_REGULATOR. Also switch to in-code check instead of ifdefs.

Fixes: 07586ee4 ("rockchip: rk3399: Support common spl_board_init")
Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com> # Rock960
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>