console: Fix rendering of VGA underline

vga_putcharxy()'s underline code sets font_data to 0xffff instead of
0xff.  vga_putcharxy() then reads dmask16[0xffff >> 4] and
dmask4[0xffff >> 6].  In practice, these out-of-bounds subscripts
"only" put a few crap bits into the display surface.

For 32 bit pixels, there's no array access.  font_data's extra bits go
straight into the display surface.

Broken when commit 6d6f7c28 implemented underline.

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
