sfc: remove redundant variable old_vlan

Variable old_vlan is being assigned but is never used hence it is
and can be removed.

Cleans up clang warning:
warning: variable 'old_vlan' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>