mlxsw: reg: Fix HTGT register length

HTGT register length is limited to 32 bytes and not 256 bytes.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>