s390/qeth: clean up qeth_l3_send_setdelmc()'s declaration

Clarify that the 'ipacmd' parameter is an enum, and thus compatible to
what qeth_ipa_alloc_cmd() expects as input.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>