net: phy: sfp: remove sfp_mutex's definition

The sfp_mutex variable is defined but never used in this file. Not even
in the commit that introduced that variable.

Remove sfp_mutex, it has no purpose.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>