mlxsw: core_env: Avoid unnecessary memcpy()s

Simply get a pointer to the data in the register payload instead of
copying it to a temporary buffer.

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