IB/mthca: Factor out setting WQE UD segment entries

Factor code to set UD entries out of the work request posting
functions into inline functions set_tavor_ud_seg() and
set_arbel_ud_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>