USB: implement "soft" unbinding

This patch (as1091) changes the way usbcore handles interface
unbinding.  If the interface's driver supports "soft" unbinding (a new
flag in the driver structure) then in-flight URBs are not cancelled
and endpoints are not disabled.  Instead the driver is allowed to
continue communicating with the device (although of course it should
stop before its disconnect routine returns).

The purpose of this change is to allow drivers to do a clean shutdown
when they get unbound from a device that is still plugged in.  Killing
all the URBs and disabling the endpoints before calling the driver's
disconnect method doesn't give the driver any control over what
happens, and it can leave devices in indeterminate states.  For
example, when usb-storage unbinds it doesn't want to stop while in the
middle of transmitting a SCSI command.

The soft_unbind flag is added because in the past, a number of drivers
have experienced problems related to ongoing I/O after their disconnect
routine returned.  Hence "soft" unbinding is made available only to
drivers that claim to support it.

The patch also replaces "interface_to_usbdev(intf)" with "udev" in a
couple of places, a minor simplification.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>