ctcm: remove BUG_ONs

Remove BUG_ON's in ctcm driver, since the checked error conditions
are null pointer accesses.

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