video/sticore: Store ROM device in STI struct

Store the ROM's parent device in each STI struct, so we can associate
the STI framebuffer with a device.

The new field will eventually replace the fbdev subsystem's info field,
which the function fb_is_primary_device() currently requires to detect
the firmware's output. By using the device instead of the framebuffer
info, a later patch can generalize the helper for use in non-fbdev code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>