qeth: clean up error handling

In the functions that are registering and unregistering MAC
addresses in the qeth-handled hardware, remove callback functions
that are unnesessary, as only the return code is analyzed.
Translate hardware response codes to semi-standard 'errno'-like
codes for readability.

Add kernel-doc description to the internal API function
qeth_send_control_data().

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Reviewed-by: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>