mlxsw: item: Add helpers for getting pointer into payload for char buffer item

Sometimes it is handy to get a pointer to a char buffer item and use it
direcly to write/read data. So add these helpers.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>