IB/mthca: Fix PRM compliance problem in atomic-send completions

According to the Tavor and Arbel programmer's reference manuals, the
number of bytes transferred is not provided in the byte_cnt field of
the CQ entry for atomic operation completions.  For atomic operations,
the number of bytes transferred is always 8 (when the status is
"success"), and this constant value should always be used by the
driver in the ib_wc entry returned, rather than using the CQE.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>