drivers/base/memory.c: introduce help macro to_memory_block

Introduce help macro to_memory_block to hide the conversion(device-->memory_block),
just clean up.

Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>