mtd: Use get_mtd_device_nm() to find named device in get_sb_mtd()

This removes the need to know the number of MTD devices.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>