sh: ms7724: modify scan_timing for KEYSC

KEYSC::SCN register of SH7724 is 3bit.
Thus, scan_timing should be 0 - 7 here.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>