enf_sf: reuse setup_flash_device instead of open coding it

setup_flash_device selects one of two code paths depending on the driver
model being used (=CONFIG_DM_SPI_FLASH). env_relocate_spec only used
the non driver-model code path. I'm unsure why, either none of the
platforms that need relocation use the driver model, or - worse - the
driver model is not yet usable when relocating.

Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>