spi: rpc-if: differentiate between unsupported and invalid requests

If the request is out of range, returning -EINVAL seems a better pick
than -ENOTSUPP.

>From a caller (and reviewer) point of view, distinguising between the
two may be helpful because somehow one can be "fixed" while the other
will always be refused no matter how hard we try.

As part of a wider work to bring spi-nand continuous reads, it was
useful to easily catch the upper limit direct mapping boundaries for
each controller, with the idea of enlarging this area from a page to an
eraseblock, without risking too many regressions.

In all other cases, as part of a wider work towards using -EOPNOTSUP
rather than -ENOTSUPP (which is not a SUSV4 code), let's change the
error code to be uniform across spi-mem controller drivers.

Finally, reword a little bit the conditions to clarify what is intended
(ie. checking for the presence of a direct mapping, and also ensuring we
create a dirmap only on DATA_IN flows).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://msgid.link/r/20240522145255.995778-4-miquel.raynal@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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