s390/qeth: reduce 0-initializing when building IPA cmds

qeth_get_ipacmd_buffer() obtains its buffers for building IPA cmds from
__qeth_get_buffer(), where they are fully cleared. So get rid of all the
additional zero-ing in various other places.

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