s390/qeth: consolidate filling of low-level cmd length fields

The code to fill the IPA length fields is duplicated three times across
the driver:
1. qeth_send_ipa_cmd() sets IPA_CMD_LENGTH, which matches the defaults
   in the IPA_PDU_HEADER template.
2. for OSN, qeth_osn_send_ipa_cmd() bypasses this logic and inserts the
   length passed by the caller.
3. SNMP commands (that can outgrow IPA_CMD_LENGTH) have their own way
   of setting the length fields, via qeth_send_ipa_snmp_cmd().

Consolidate this into qeth_prepare_ipa_cmd(), which all originators of
IPA cmds already call during setup of their cmd. Let qeth_send_ipa_cmd()
pull the length from the cmd instead of hard-coding IPA_CMD_LENGTH.

For now, the SNMP code still needs to fix-up its length fields manually.

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