s390/qeth: simplify qeth_receive_skb()

Now that the OSN code is gone, we don't need the second switch statement
in the RX path. And getting rid of its (unreachable) default case is a
nice simplification.

Also don't pass in the full HW header, all we still need is a flag to
indicate whether the skb can use CSO. This we can already obtain during
the first peek at the HW header.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>