Fix possible NULL pointer dereference in case devm_kzalloc() failed to
allocate memory

Fixes: cc13e52c ("net: dsa: microchip: Add Microchip KSZ8863 SPI based driver support")
Reported-by: Colin Ian King <colin.king@canonical.com>
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>