RDMA/cxgb4: Don't exceed hw IQ depth limit for user CQs

Memory allocated for user CQs gets rounded up to the next page
boundary.  And after rounding, we recalculate the resulting IQ depth
and we need to make sure we don't exceed the HW limits.

This bug can result a much smaller CQ allocated than was expected if
the HW size field is exceeded, resulting in CQ overflow failures.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>