RDMA/cxgb3: Fix too-big reserved field zeroing in iwch_post_zb_read()

The array wqe->read.reserved has only two entries, but
iwch_post_zb_read() sets [0], [1], and [2], which is one too many.
This is harmless since it runs into the next field, rem_stag, which is
initialized correctly immediately after, but we might as well get
things right, especially since it makes the code smaller.

This was spotted by the Coverity checker (CID 2475).

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>