x86, setup: fix the setting of 480-line VGA modes

Impact: fix rarely-used feature

The VGA Miscellaneous Output Register is read from address 0x3CC but
written to address 0x3C2.  This was missed when this code was
converted from assembly to C.  While we're at it, clean up the code by
making the overflow bits and the math used to set the bits explicit.

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