common: env_sf: Use CONFIG_SF_DEFAULT_xxx as the default value for CONFIG_ENV_SPI_xxx

The default values for the configuration defines CONFIG_ENV_SPI_xxx are
arbitrary values. It makes more sense to set them to the values used by
the sf command.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>