USB: fix usb_serial_suspend(): buggy code

Am Montag 23 Juli 2007 schrieb Adrian Bunk:
> Commit ec22559e added the following 
> function to drivers/usb/serial/usb-serial.c:
> 
[..]
> 
> The Coverity checker spotted the inconsequent NULL checking for "serial".
> 
> Looking at the code it also doesn't seem to have been intended to always 
> return 0.

Coverity is right. The check for NULL is wrongly done and the error
return is lost.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>