btl/usnic: set ack_iteration_delay default to 4

It was previously accidentally set to 0.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
(cherry picked from commit 132e4cab3bc71df0da87368a332d6af0090a6977)

opal/mca/btl/usnic/btl_usnic_mca.c

@@ -265,7 +265,7 @@ int opal_btl_usnic_component_register(void)
                   REGINT_GE_ONE, OPAL_INFO_LVL_5));
 
     CHECK(reg_int("ack_iteration_delay", "Minimum number of times through usNIC \"progress\" function before checking to see if standalone ACKs need to be sent",
-                  0, &mca_btl_usnic_component.ack_iteration_delay,
+                  4, &mca_btl_usnic_component.ack_iteration_delay,
                   REGINT_GE_ZERO, OPAL_INFO_LVL_5));
 
     CHECK(reg_int("priority_limit", "Max size of \"priority\" messages (0 = use pre-set defaults; depends on number and type of devices available)",