net/sched: cbs: remove redundant assignment to variable port_rate

Variable port_rate is being initialized with a value that is never read
and is being re-assigned a little later on. The assignment is redundant
and hence can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>