xircom_cb should return NETDEV_TX_BUSY when no descriptors available

Changes in other networking paths uncovered a bug in the xircom_cb
driver which made the kernel spew lots of the following error messages:

  BUG eth1 code -5 qlen 0

It turned out that the driver returned -EIO when there was no
descriptor available for sending packets. It should return
NETDEV_TX_BUSY instead. This was discussed on the netdev list before,
see http://thread.gmane.org/gmane.linux.network/84603 .

Signed-off-by: Erik Mouw <mouw@nl.linux.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>