cxgb4: Deal with wrap-around of queue for Work request

The WR headers  may not fit within one descriptor.
So we need to deal with wrap-around here.

Based on original patch by Pranjal Joshi <pjoshi@chelsio.com>

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>