USB: Sierra: fix oops upon device close

This patch (as1263) fixes a mixup that occurred when conflicting
patches for the sierra driver were merged incorrectly.  The former
sierra_shutdown routine should have been become sierra_release, not
sierra_disconnect.

The symptom this fixes is an oops when the device file is closed after
a Sierra device has been unplugged (Bugzilla #13675).

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Peter Naulls <peter@mushroomnetworks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>