staging: exfat: add STAGING prefix to config names

Add STAGING prefix to config names to avoid collsion with fs/exfat config.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Link: https://lore.kernel.org/r/20200103011345.25245-1-namjae.jeon@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>