sfc: initialise found bitmap in efx_ef10_mtd_probe

The bitmap of found partitions in efx_ef10_mtd_probe was not
initialised, causing partitions to be suppressed based off whatever
value was in the bitmap at the start.

Fixes: 33664635

 ("sfc: suppress duplicate nvmem partition types in efx_ef10_mtd_probe")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>