dm: env_sf: fix saveenv() to use driver model

It might be missed when converting spi_flash_probe() in cmd_sf.c.

This patch refers to commit fbb09918 ("dm: Convert
spi_flash_probe() and 'sf probe' to use driver model").

Signed-off-by: Gong Qianyu <Qianyu.Gong@freescale.com>
Reviewed-by: York Sun <york.sun@nxp.com>