USB: unify some error pathways in usbfs

This patch (as1496) unifies the error-return pathways of several
functions in the usbfs driver.  This is not a very important change by
itself; it merely prepares the way for the next patch in this series.

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