Documentation: DMA-API: fix a function name of max_mapping_size

The exported function name is dma_max_mapping_size(), not
dma_direct_max_mapping_size() so that this patch fixes
the function name in the documentation.

Fixes: 133d624b ("dma: Introduce dma_max_mapping_size()")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>