IB/mthca: Check that SRQ WQE size does not exceed device's max value

Guarantee the calculated work queue entry size does not exceed the max
allowable WQE size when creating an SRQ.  This is a problem with Arbel
in Tavor-compatibility mode because the current WQE size computation
method rounds up to next power of 2.

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