net: dsa: microchip: Dispose of ksz_io_ops

Since the driver now uses regmap , get rid of ad-hoc ksz_io_ops
abstraction, which no longer has any meaning. Moreover, since regmap
has it's own locking, get rid of the register access mutex.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Tristram Ha <Tristram.Ha@microchip.com>
Cc: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>