s390/qeth: move netif_napi_add_tx() and napi_enable() from under BH

Like other drivers qeth is calling local_bh_enable() after napi_schedule()
to kick-start softirqs [0].
Since netif_napi_add_tx() and napi_enable() now take the netdev_lock()
mutex [1], move them out from under the BH protection. Same solution as in
commit a6055864 ("wifi: mt76: move napi_enable() from under BH")

Fixes: 1b23cdbd ("net: protect netdev->napi_list with netdev_lock()")
Link: https://lore.kernel.org/netdev/20240612181900.4d9d18d0@kernel.org/ [0]
Link: https://lore.kernel.org/netdev/20250115035319.559603-1-kuba@kernel.org/

 [1]
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Acked-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250212163659.2287292-1-wintera@linux.ibm.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>