enetc_alloc_txbdr() deals with allocating resources necessary for a TX
ring to work (the array of software BDs and the array of TSO headers).

The next_to_clean and next_to_use pointers are overwritten with proper
values which are read from hardware here:

enetc_open
-> enetc_alloc_tx_resources
   -> enetc_alloc_txbdr
      -> set to zero
-> enetc_setup_bdrs
   -> enetc_setup_txbdr
      -> read from hardware

So their initialization with zeroes is pointless and confusing.
Delete it.

Consequently, since enetc_setup_txbdr() has no opposite cleanup
function, also delete the resetting of these indices from
enetc_free_tx_ring().
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>