mmc: atmel-mci: fix return value check of mmc_add_host()

In the Linux kernel, the following vulnerability has been resolved: mmc: atmel-mci: fix return value check of mmc_add_host() mmc_add_host() may return error, if we ignore its return value, it will lead two issues: 1. The memory that allocated in mmc_alloc_host() is leaked. 2. In the remove() path, mmc_remove_host() will be called to delete device, but it's not added yet, it will lead a kernel crash because of null-ptr-deref in device_del(). So fix this by checking the return value and calling mmc_free_host() in the error path.