allow adbd to set sys.usb.ffs.ready

Needed for https://android-review.googlesource.com/147730

Change-Id: Iceb87f210e4c5d0f39426cc6c96a216a4644eaa9
diff --git a/property_contexts b/property_contexts
index 06f6c17..b284197 100644
--- a/property_contexts
+++ b/property_contexts
@@ -20,6 +20,7 @@
 hw.                     u:object_r:system_prop:s0
 sys.                    u:object_r:system_prop:s0
 sys.powerctl            u:object_r:powerctl_prop:s0
+sys.usb.ffs.            u:object_r:ffs_prop:s0
 service.                u:object_r:system_prop:s0
 wlan.                   u:object_r:system_prop:s0
 dhcp.                   u:object_r:dhcp_prop:s0