nfp: use struct fields for 8 bit-wide access

Use direct access struct fields rather than PREP_FIELD()
macros to manipulate the jump ID and length, both of which
are exactly 8-bits wide. This simplifies the code somewhat.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>