init: usb: disable usb function switch until adbd is ready

In the case of adb enabled, When a usb function switch happens
e.g mtp,adb -> ptp,adb, framework will set sys.usb.config from
"mtp,adb" to "none" and stop adbd first.
At this time, the property "sys.usb.ffs.ready", which is set after
the ready of the perious start adbd, is still "1".
So , when framework goes on setting the sys.usb.config to "ptp,adb",
init.usb.configfs.rc will trigger actions to bind() f_fs without
waiting for adbd is actually ready.

This will produce some error messages:
------------
configfs-gadget ci_hdrc.0: failed to start g1: -19
init: write_file: Unable to write to '/config/usb_gadget/g1/UDC': No such device
------------
this error msg is caused by configFS start binding f_fs before adbd is ready.

Add setting the "sys.usb.ffs.ready" to "0" will help clear this error msg,
as the bind process in kernel gadget driver will wait for the set of
this property, which is the sign of adbd is ready.

Signed-off-by: Winter Wang <wente.wang@nxp.com>