i2o: core: added i2o_data_xfer

i2o_data_xfer is the function which will be responsible for handling
data transmissions to and from I/O chips. Added a stub only.

Signed-off-by: Michel Megens <dev@michelmegens.net>
i2o: core: implemented a mechanism to lock devs.

Chips (aka, devices) can now be 'requested', which means to can't
disappear into oblivion anymore. Therefore it is safe to perform
operations ONLY after having called i2o_dev_request.

Signed-off-by: Michel Megens <dev@michelmegens.net>
i2o: core: renamed i2o_data_xfer to i2o_dev_xfer.

i2o_data_xfer is from now on called i2o_dev_xfer to describe its use
better. I've also added a function which is visible outside of the
I2O-core module to make use of this functionality.

Signed-off-by: Michel Megens <dev@michelmegens.net>