mlxsw: reg: Drop SGCR.llb

SGCR, Switch General Configuration Register, has not been used since commit
b0d80c01 ("mlxsw: Remove Mellanox SwitchX-2 ASIC support"). We will
need the register again shortly, so instead of dropping it and
reintroducing again, just drop the sole unused field.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>