mwifiex: only call mwifiex_sdio_probe_of() if dev has an OF node

SDIO is an auto enumerable bus so the SDIO devices are matched using the
sdio_device_id table and not using compatible strings from a OF id table.

However, commit ce4f6f0c ("mwifiex: add platform specific wakeup
interrupt support") allowed to match nodes defined as child of the SDIO
host controller in the probe function using a compatible string to setup
platform specific parameters in the DT.

The problem is that the OF parse function is always called regardless if
the SDIO dev has an OF node associated or not, and prints an error if it
is not found. So, on a platform that doesn't have a node for a SDIO dev,
the following misleading error message will be printed:

[  12.480042] mwifiex_sdio mmc2:0001:1: sdio platform data not available

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>