mmc: mxcmmc: fix return value check of mmc_add_host()

In the Linux kernel, the following vulnerability has been resolved: mmc: mxcmmc: fix return value check of mmc_add_host() mmc_add_host() may return error, if we ignore its return value, the memory that allocated in mmc_alloc_host() will be leaked and it will lead a kernel crash because of deleting not added device in the remove path. So fix this by checking the return value and goto error path which will call mmc_free_host().