net: pch_gbe: Use 'dma_free_coherent()' to undo 'dma_alloc_coherent()'

Memory allocation are done with 'dma_alloc_coherent()'. Be consistent
and use 'dma_free_coherent()' to free the corresponding memory.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/20201121090330.1332543-1-christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>