x86: zImage: Move subarch assignment out of cmd_line check

The commit

  20bfac05 ("x86: zImage: add Intel MID platforms support")

introduced an assignment of subarch field in boot parameters, though
missed the right place of doing that. It doesn't matter if we have or
not a kernel command line supplied, we just set that field. Although
guard it by protocol version which supports it.

Fixes: 20bfac05 ("x86: zImage: add Intel MID platforms support")
Cc: Vincent Tinelli <vincent.tinelli@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>