sfc: suppress duplicate nvmem partition types in efx_ef10_mtd_probe

Use a bitmap to keep track of which partition types we've already seen;
 for duplicates, return -EEXIST from efx_ef10_mtd_probe_partition() and
 thus skip adding that partition.
Duplicate partitions occur because of the A/B backup scheme used by newer
 sfc NICs.  Prior to this patch they cause sysfs_warn_dup errors because
 they have the same name, causing us not to expose any MTDs at all.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>