net: sfp: Fix typo in state machine debug string

The string should be "tx_disable" to match the state enum.

Fixes: 4005a7cb

 ("net: phy: sftp: print debug message with text, not numbers")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>