s390/qeth: limit trace to valid data of command request

'len' specifies how much data we send to the HW, don't dump beyond this
boundary.
As of today this is no big concern - commands are built in full, zeroed
pages.

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