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.

Fixes: 4e268fed

 ("mmc: Add mmc driver for Sunplus SP7021")
Cc: stable@vger.kernel.org
Signed-off-by: Wei Chen <harperchen1110@gmail.com>
Link: https://lore.kernel.org/r/20230622090233.188539-1-harperchen1110@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>