nfp: bpf: pass dst register to ld_field instruction

ld_field instruction is a bit special because the encoding uses
two source registers and one of them becomes the output.  We do
need to pass the dst register to our encoding helpers though,
otherwise the "write both banks" flag will not be observed.

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