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

In the Linux kernel, the following vulnerability has been resolved:

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

mmc_add_host() may return error, if we ignore its return value,

1. the memory allocated in mmc_alloc_host() will be leaked

2. null-ptr-deref will happen when calling mmc_remove_host()

in remove function spmmc_drv_remove() because deleting not

added device.

Fix this by checking the return value of mmc_add_host(). Moreover,

I fixed the error handling path of spmmc_drv_probe() to clean up.