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.