fbdev: stifb: Make the STI next font pointer a 32-bit signed offset

The pointer to the next STI font is actually a signed 32-bit
offset. With this change the 64-bit kernel will correctly subract
the (signed 32-bit) offset instead of adding a (unsigned 32-bit)
offset. It has no effect on 32-bit kernels.

This fixes the stifb driver with a 64-bit kernel on qemu.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org