ldlinux: Don't update UsingVGA when we just want the value

ldlinux: Don't update UsingVGA when we just want the value

There was a bug introduced when this code was copied from the old 4.x
assembly implementation, we don't need to update UsingVGA everytime we
read a character, we only need its value when comparing it against the
DisplayMask.

This bug caused every other character to be displayed when only
writing to the text console, as well as other nasties such as making
syslinux_force_text_mode() think it's in VGA mode even when it's in
text mode.

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