iw_cxgb4: fix for 64-bit integer division

Fixed error introduced in commit id 7730b4c (" cxgb4/iw_cxgb4: work request
logging feature") while compiling on 32 bit architecture reported by kbuild.

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