wlcore: use PLATFORM_DEVID_AUTO for plat dev creation to avoid conflicts

The platform devices can be created by both wlcore_sdio and
wlcore_spi.  Theoretically, if both are connected to the same board,
there will be a conflict.

Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>