env: sf: Put ENV_INVALID into gd->env_valid on CRC failure

env_sf_init_addr() says the environment is valid even if it is assigning
default environment due to CRC failure. Change this to ENV_INVALID and
let the generic env_init() function, which calls this initializer,
assign the default environment.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>