sfc: Clear RXIN_SEL when soft-resetting QT2025C

When we enable PMA/PMD loopback this automatically sets RXIN_SEL
(inverse polarity for RXIN).  We need to clear that bit during the
soft-reset sequence, as it is not done automatically.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>