IB/ehca: Return correct number of SGEs for SRQ

Firmware would round up the number of SGEs to four, because the WQE
structure holds four SGEs. For SRQ, only three are supported, so return
a fixed value instead.

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