nfp: bpf: fix immed relocation for larger offsets

Immed relocation is missing a shift which means for larger
offsets the lower and higher part of the address would be
ORed together.

Fixes: ce4ebfd8

 ("nfp: bpf: add helpers for updating immediate instructions")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>