nfp: read chip model from the PluDevice register

The PluDevice register provides the authoritative chip model/revision.

Since the model number is purely used for reporting purposes, follow
the hardware team convention of subtracting 0x10 from the PluDevice
register to obtain the chip model/revision number.

Suggested-by: Francois H. Theron <francois.theron@netronome.com>
Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>