video: bochs: Drop the useless argument of bochs_vga_write()

bochs_vga_write() takes 'index' as one argument, but never uses it.

While we are here, use macros instead of magic numbers for the
VGA IO port register name and value.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # qemu-x86_64