mlxsw: spectrum: Do not override PAUSE settings

The PFCC register is used to configure both PAUSE and PFC frames.
Therefore, when PFC frames are disabled we must make sure we don't
mistakenly also disable PAUSE frames (which might be enabled).

Fix this by packing the PFCC register with the current PAUSE settings.

Note that this register is also accessed via ethtool ops, but there we
are guaranteed to have PFC disabled.

Fixes: d81a6bdb ("mlxsw: spectrum: Add IEEE 802.1Qbb PFC support")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>