IB/ehca: Beautify sysfs attribute code and fix compiler warnings

eHCA's sysfs attributes are now being created via sysfs_create_group(),
making the process neatly table-driven. The return value is checked, thus
fixing a few compiler warnings.

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