nfp: fix simple vNIC mailbox length

The simple vNIC mailbox length should be 12 decimal and not 0x12.
Using a decimal also makes it clear this is a length value and not
another field within the simple mailbox defines.

Found by code inspection, there are no known firmware configurations
where this would cause issues.

Fixes: 527d7d1b ("nfp: read mailbox address from TLV caps")
Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>