et131x: Remove useless DMA-32 fallback configuration

et131x: Remove useless DMA-32 fallback configuration

As stated in [1], dma_set_mask() with a 64-bit mask never fails if
dev->dma_mask is non-NULL.
So, if it fails, the 32 bits case will also fail for the same reason.

Moreover, dma_set_mask_and_coherent() returns 0 or -EIO, so the return
code of the function can be used directly. There is no need to 'rc = -EIO'
explicitly.

Simplify code and remove some dead code accordingly.

[1]: https://lkml.org/lkml/2021/6/7/398

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/b9aa46e7e5a5aa61f56aac5ea439930f41ad9946.1641726804.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>