env: sf: remove the static env_flash variable

As the the SPI flash is probed and is released in each ENV sf function
the env_flash no more need to be static.

This patch move this device handle as local variable of each function and
simplify the associated code (env_flash is never == NULL when
setup_flash_device is called).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>