net: ethernet: lantiq_etop: Fix compilation error

This fixes the error detected when compiling the driver.

Fixes: 14d4e308 ("net: lantiq: configure the burst length in ethernet drivers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>