ethtool: fec_prepare_data() - jump to error handling

Refactor fec_prepare_data() a little bit to skip the body
of the function and exit on error. Currently the code
depends on the fact that we only have one call which
may fail between ethnl_ops_begin() and ethnl_ops_complete()
and simply saves the error code. This will get hairy with
the stats also being queried.

No functional changes.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>