mtd: Fix handling of mtdname in txx9ndfmc.c

As pointed out by Kay Sievers, the name size limit is gone
from the driver-core, and BUS_ID_SIZE is obsolescent.

Rather than just papering over the problem by replacing the mtdname
array size with an arbitrary '20 + 2', fix the problem properly and
handle arbitrary name sizes.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>