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

There's only a check if mwifiex_add_card() returned a nonzero value, but
the actual error code is neither stored nor propagated to the caller. So
instead of always returning -1 (which is -EPERM and not a suitable errno
code in this case), propagate the value returned by mwifiex_add_card().

Patch also removes the assignment of sdio_disable_func() returned value
since it was overwritten anyways and what matters is to know the error
value returned by the first function that failed.

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>