mwifiex: check if mwifiex_sdio_probe_of() fails and return error

The function can fail so the returned value should be checked
and the error propagated to the caller in case of a failure.

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>