pcspkr: fix dependancies

fix pcspkr dependancies: make the pcspkr platform
drivers to depend on a platform device, and
not the other way around.

Signed-off-by: Stas Sergeev <stsp@aknet.ru>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
CC: Vojtech Pavlik <vojtech@suse.cz>
CC: Michael Opdenacker <michael-lists@free-electrons.com>
[fixed for 2.6.26-rc1 by tiwai]
Signed-off-by: Takashi Iwai <tiwai@suse.de>