cxgb4vf: Fix off-by-one error checking for the end of the mailbox delay array

Fix off-by-one error in checking for the end of the mailbox response delay
array.  We ended up walking off the end and, if we were unlucky, we'd end up
pulling in a 0 and never terminate the mailbox response delay loop ...

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>