[PATCH] USB: gadget-serial: do not save/restore IRQ flags in gs_close()

As pointed out by David Brownell, we know that IRQs are never
blocked when calling gs_close function. So the save/restore
IRQ flags are pointless.

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>