RDMA/hns: Prevent undefined behavior in hns_roce_set_user_sq_size()

The "ucmd->log_sq_bb_count" variable is a user controlled variable in the
0-255 range.  If we shift more than then number of bits in an int then
it's undefined behavior (it shift wraps), and potentially the int could
become negative.

Fixes: 9a443537

 ("IB/hns: Add driver files for hns RoCE driver")
Link: https://lore.kernel.org/r/20190608092514.GC28890@mwanda
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>