m68k: Fix unused variable in board.c

The following warning was produced, fix it:

board.c: In function 'board_init_r':
board.c:390:8: warning: unused variable 's' [-Wunused-variable]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: "Jin Zhengxiong-R64188" <R64188@freescale.com>
Cc: Jason Jin <jason.jin@freescale.com>