net/hsr: Remove unused but set variable

Remove the unused but set variable master_dev in check_local_dest to fix
the following GCC warning when building with 'W=1':

  net/hsr/hsr_forward.c: In function ‘check_local_dest’:
  net/hsr/hsr_forward.c:303:21: warning: variable ‘master_dev’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>