rawcon_read: Fix reading high part of input keys

rawcon_read: Fix reading high part of input keys

commit 8486142cf304 ("elflink: Replace __intcall() with direct
function calls") made the mistake of zero'ing the high part of the
input key on every invocation of __rawcon_read() instead of preserving
it across calls like the old code, which lead to function keys such as
KEY_UP, KEY_DOWN, etc not working.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>