[MMC] wbsd: convert to the new platfrom device interface

platform_device_register_simple() is going away, switch to
using platfrom_device_alloc() + platform_device_add(). Also
make sure that wbsd_driver gets unregistered when wbsd_init
fails.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Acked-by: Pierre Ossman <drzeus@drzeus.cx>