net/mlx5: update modify field action

net/mlx5: update modify field action

Update immediate value/pointer source operand support
for modify field RTE Flow action:

  - source operand data can be presented by byte buffer
    (instead of former uint64_t) or by pointer
  - no host byte ordering is assumed anymore for immediate
    data buffer (not uint64_t anymore)
  - no immediate value offset is expected (the source
    subfield is located at the same offset as in destination)

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>