Staging: rtl8723bs: fix an error code in isFileReadable()

The caller only cares about zero vs non-zero so this code actually works
fine but we should be returning a negative error code instead of a valid
pointer casted to int.

Fixes: 554c0a3a ("staging: Add rtl8723bs sdio wifi driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>