bios: Fix lowmem check

The lowmem check was backwards and would report error when there
wasn't any.  This was missed because the Ctrl key check was *also*
backwards and so the check would only be done when Ctrl was held down.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
bios: Don't truncate memory size needed to 16 bits

We can't truncate the memory size needed to 16 bits *before* we
convert it to kilobytes...

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>