s390/qeth: simplify fragment type selection

Improve readability of the code that determines a buffer element's
fragment type, and reduce the number of cases down from 5 to 3.

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