net: bridge: remove redundant assignment

The variable br is assigned a value that is not being read after
exiting case IFLA_STATS_LINK_XSTATS_SLAVE. The assignment is
redundant and can be removed.

Addresses-Coverity ("Unused value")
Signed-off-by: Nigel Christian <nigel.l.christian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>