s390/qeth: streamline SNMP cmd code

Apply some cleanups to qeth_snmp_command() and its callback:
1. when accessing the user data, use the proper struct instead of
   hard-coded offsets. Also copy the request data straight into the
   allocated cmd, skipping the extra memdup_user() to a tmp buffer.
2. capping the request length is no longer needed, the same check gets
   applied at a base level in qeth_alloc_cmd().
3. clean up some duplicated (and misindented) trace statements.

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