net: dsa: sja1105: Export the sja1105_inhibit_tx function

This will be used to stop egress traffic in .phylink_mac_link_up.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>