nfp: use PluDevice register for model for non-NFP6000 chips

The model number for NFP3800 and newer devices can be completely
derived from PluDevice register without subtracting 0x10.

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>