env_sf: updated to the new environment code

Functions to store/retrieve the environment from a SPI flash was not updated
to the new environment code. The non-redundant case was
not working correctly, reporting ""Environment SPI flash not initialized"
and the code was not compiled clean in the redundant case.

The patch fixes these issue and makes the code more coherent
with other environment storage (nand, flash).

Signed-off-by: Stefano Babic <sbabic@denx.de>