drivers/tty/serial/sh-sci.c: suppress warning

drivers/tty/serial/sh-sci.c: In function sci_dma_rx_submit:
./include/linux/spinlock.h:288:3: warning: flags may be used uninitialized in this function [-Wmaybe-uninitialized]
   _raw_spin_unlock_irqrestore(lock, flags); \
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/tty/serial/sh-sci.c:1353:16: note: flags was declared here
  unsigned long flags;

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>