RDMA/cxgb3: Clean up signed check of unsigned variable

Q_FREECNT() returns the number of spaces free.  This should never be a
negative amount.  Also the num_wrs is an unsigned int so it can never
be less than zero.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>