IB/mthca: Factor out setting WQE remote address and atomic segment entries

Factor code to set remote address and atomic segment entries out of the
work request posting functions into inline functions set_raddr_seg()
and set_atomic_seg().  This doesn't change the generated code in any
significant way, and makes the source easier on the eyes.

Signed-off-by: Roland Dreier <rolandd@cisco.com>