et131x: Add missing check after DMA map

[ Upstream commit d61f6cb6f6ef3c70d2ccc0d9c85c508cb8017da9 ]

The DMA map functions can fail and should be tested for errors.
If the mapping fails, unmap and return an error.

Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Acked-by: Mark Einon <mark.einon@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250716094733.28734-2-fourier.thomas@gmail.com


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