i2o: core: implemented i2o_channel_xfer.

i2o_channel_xfer is a prototype and standard function assigned to
channels on creation by i2o_channel_alloc. I/O chip drivers can
overwrite this function pointer (struct i2o_channel::xfer) with a
pointer to a driver specific function if special behaviour is required.

This function is some sort of an 'in-between' function. The only
functionality it has is calling the right read/write functions which are
stored in the static info structure (struct i2o_static, struct
i2o_dev::info).

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