nfp: choose data path based on version

Prepare for choosing data path based on the firmware version field.
Exploit one bit from the reserved byte in the firmware version field
as the data path type.  We need the firmware version right after
vNIC is allocated, so it has to be read inside nfp_net_alloc(),
callers don't have to set it afterwards.

Following patches will bring the implementation of the second data
path.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>