env: sf: Free the old env_flash

env_flash is a global flash pointer, and the probe would
happen only if env_flash is NULL, but there is no checking
and free the pointer if is not NULL.

So, this patch frees the old env_flash, and get the probed
flash in to env_flash pointer and finally check if is not NULL.

Cc: Simon Glass <sjg@chromium.org>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>