IB/ehca: Allocate event queue size depending on max number of CQs and QPs

If a lot of QPs fall into Error state at once and the EQ of the
respective HCA is too small, it might overrun, causing the eHCA driver
to stop processing completion events and calling the application's
completion handlers, effectively causing traffic to stop.

Fix this by limiting available QPs and CQs to a customizable max
count, and determining EQ size based on these counts and a worst-case
assumption.

Signed-off-by: Stefan Roscher <stefan.roscher@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>