RDMA/amso1100: Check for integer overflow in c2_alloc_cq_buf()

This is a static checker fix.  The static checker says that q_size comes
from the user and can be any 32 bit value.  The call tree is:
  --> ib_uverbs_create_cq()
      --> c2_create_cq()
          --> c2_init_cq()

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>