IB/mthca: Fix initial SRQ logsize for mem-free HCAs

When initializing an mthca SRQ, the log_srq_size field should be the
log of the number of SRQ WQEs, not the log of the number of bytes in
the SRQ.

This affects only mthca drivers for memfree HCAs which set the initial
srq wqe counter (in the SW2HW transition) to a non-zero value.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>