s390/qeth: stop yielding the ip_lock during IPv4 registration

As commit df2a2a52 ("s390/qeth: convert IP table spinlock to mutex")
converted the ip_lock to a mutex, we no longer have to yield it while
the subsequent IO sleep-waits for completion.

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