IB/ehca: Use common error code mapping instead of specific ones

Instead of one error mapping function for each potential error source
in ehca_mrmw.c, use a centralized function that handles all cases,
saving a three-figure line count.

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