net: alteon: remove unused len variable

clang with W=1 reports
drivers/net/ethernet/alteon/acenic.c:2438:10: error: variable
  'len' set but not used [-Werror,-Wunused-but-set-variable]
                int i, len = 0;
                       ^
This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>