usb_serial: Kill port mutex

The tty port has a port mutex used for all the port related locking so we
don't need the one in the USB serial layer any more.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>