drivers/misc/phantom: note PCI

Tell users that the driver is only for PCI devices to stop asking for
support of firewire and parallel devices.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>