_exports.h: simplify condition for including spi functions

_exports.h: simplify condition for including spi functions

As for the i2c functions, drop the dummy entries that, if ever used,
would just have the standalone app get some random content in the
return register.

While deprecated, the spi_{setup,free}_slave functions do exist even
with CONFIG_DM_SPI - and a standalone app can't really do anything but
refer to a spi device via a (bus, cs) pair.

Eventually, one should probably export some function that could allow
a standalone app to get a struct udevice* corresponding to either a
full DT path, an alias, or perhaps a label (provided one builds with
-@), and then export functions that can operate on that.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>