bridge: reset bridge mtu after deleting an interface

On adding an interface br_add_if() sets the MTU to the min of
all the interfaces. Do the same thing on removing an interface too
in br_del_if.

Signed-off-by: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>