net: enetc: rx_swbd and tx_swbd are never NULL in enetc_free_rxtx_rings()

The call path in enetc_close() is:

enetc_close()
-> enetc_free_rxtx_rings()
   -> enetc_free_rx_ring()
      -> tests whether rx_ring->rx_swbd is NULL
   -> enetc_free_tx_ring()
      -> tests whether tx_ring->tx_swbd is NULL
-> enetc_free_rx_resources()
   -> enetc_free_rxbdr()
      -> sets rxr->rx_swbd to NULL
-> enetc_free_tx_resources()
   -> enetc_free_txbdr()
      -> setx txr->tx_swbd to NULL

From the above, it is clear that due to the function ordering, the
checks for NULL are redundant, since the software buffer descriptor
arrays have not yet been set to NULL. Drop these checks.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>