net: devmem: fix DMA direction on unmapping

Looks like we always unmap the DMA_BUF with DMA_FROM_DEVICE direction.
While at it unexport __net_devmem_dmabuf_binding_free(), it's internal.

Found by code inspection.

Fixes: bd618489 ("net: devmem: Implement TX path")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250801011335.2267515-1-kuba@kernel.org


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