s390/mem_detect: Use IS_ENABLED(CONFIG_BLK_DEV_INITRD)

IS_ENABLED should generally use CONFIG_ prefaced symbols and
it doesn't appear as if there is a BLK_DEV_INITRD define.

Cc: <stable@vger.kernel.org> # 4.20
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>