mmc: fix compile error when CONFIG_BLOCK is not enabled

'DISK_NAME_LEN' is undeclared when CONFIG_BLOCK is disabled; its use was
introduced via genhd.h by the general purpose partition patch.

To fix, we just add our own MAX_MMC_PART_NAME_LEN macro instead of using
DISK_NAME_LEN.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Andrei Warkentin <andreiw@vmware.com>
Signed-off-by: Chris Ball <cjb@laptop.org>