hsr: remove unnecessary code in hsr_dev_change_mtu()

In the hsr_dev_change_mtu(), the 'dev' and 'master->dev' pointer are
same. So, the 'master' variable and some code are unnecessary.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>