s390/qeth: Fix error handling during VNICC initialization

Smatch discovered the use of uninitialized variable sup_cmds
in error paths.

Fixes: caa1f0b1 ("s390/qeth: add VNICC enable/disable support")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>