mfd: rk808: Fix up the chip id get failed

the rk8xx chip id is:
((MSB << 8) | LSB) & 0xfff0

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Signed-off-by: Joseph Chen <chenjh@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>