mwifiex: propagate sdio_enable_func() errno code in mwifiex_sdio_probe()

If the sdio_enable_func() function fails on .probe, the -EIO errno code
is always returned but that could make more difficult to debug and find
the cause of why the function actually failed.

Since the driver/device core prints the value returned by .probe in its
error message propagate what was returned by sdio_enable_func() at fail.

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>