Originally the cpuid var the value gets read into was defined as
    u8 cpuid[RK3399_CPUID_LEN];
hence the sizeof(cpuid) would return the correct the correct number
of array elements.

With the move to a separate function cpuid becomes a pointer and
sizeof(cpuid) hence returns the pointer size - 8 in the arm64 case.

We do have the actual id length available as function param so use
it for actual amount of bytes to read.

Fixes: 04825384 ("rockchip: rk3399: derive ethaddr from cpuid")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>