net: dsa: ksz: ksz8863_smi_probe: set proper return value for ksz_switch_alloc()

ksz_switch_alloc() will return NULL only if allocation is failed. So,
the proper return value is -ENOMEM.

Fixes: 60a36476

 ("net: dsa: microchip: Add Microchip KSZ8863 SMI based driver support")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>