qeth: Remove BUG_ONs

Remove BUG_ONs or convert to WARN_ON_ONCE/WARN_ONs since a failure within a
networking device driver is no reason to shut down the entire machine.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Reviewed-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>