IB/ehca: HW level, HW caps and MTU autodetection

In preparation for support of new eHCA2 features, change adapter probing:
 - Hardware level is changed to encode major and minor chip version
 - Hardware capabilities are queried from the firmware
 - The maximum MTU is queried from the firmware instead of assuming a
   fixed value

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