net/mlx4: tidy up flow rule handling code

net/mlx4: tidy up flow rule handling code

- Remove unnecessary casts.
- Replace consecutive if/else blocks with switch statements.
- Use proper big endian definitions for mask values.
- Make end marker checks of item and action lists less verbose since they
  are explicitly documented as being equal to 0.
- Remove unnecessary NULL check on action configuration structure.

This commit does not cause any functional change.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>