usb-skeleton: don't submit URBs after disconnection

This patch (as712b) is a slight revision of one submitted earlier.  It
fixes the usb-skeleton example driver so that it won't try to submit
URBs after skel_disconnect() has returned.  This could cause errors, if
the driver was unbound and then a different driver was bound to the
device.  It also fixes a couple of small bugs in the skel_write()
routine.

The revised patch uses a slightly different test, suggested by Dave
Brownell, for determining whether to free a transfer buffer.  It's a
little clearer than the earlier version.


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