net: dsa: qca: ar9331: drop pointless static qualifier in ar9331_sw_mbus_init

There is no need to set variable 'mbus' static
since new value always be assigned before use it.

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>