IB/ehca: Move ehca2ib_return_code() out of line

ehca2ib_return_code() is not used in any fast path, and making it
non-inline saves ~1.5K of code.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>