Merge branch 'mlxsw-spectrum-Implement-loopback-ethtool-feature'

Ido Schimmel says:

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mlxsw: spectrum: Implement loopback ethtool feature

This patchset from Jiri allows users to enable loopback feature for
individual ports using ethtool. The loopback feature is useful for
testing purposes and will also be used by upcoming patchsets to enable
the monitoring of buffer drops.

Patch #1 adds the relevant device register.

Patch #2 Implements support in the driver.

Patch #3 adds a selftest.
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Signed-off-by: David S. Miller <davem@davemloft.net>