core: remove dev->d_op, introduce cset->raw_io instead

When a device has several input or output csets, we usually want
a different data-transfer function for each of them. Using a function
specific to each cset simplifies things a little.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>