cris: fix a build error in sync_serial_open()

cris: fix a build error in sync_serial_open()

Fix:

  arch/cris/arch-v10/drivers/sync_serial.c:628: error: 'ret' undeclared (first use in this function)

'ret' should be 'err'.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>