mmc_alloc_host() returns NULL pointer not PTR_ERR(), if it
fails, so replace the IS_ERR() check with NULL pointer check.

In commit 418f7c2d ("mmc: meson-gx: use devm_mmc_alloc_host"),
it checks NULL pointer not PTR_ERR, if devm_mmc_alloc_host() fails,
so make it to return NULL pointer to keep same with mmc_alloc_host(),
the drivers don't need to change the error handle when switch to
use devm_mmc_alloc_host().

Fixes: 80df83c2 ("mmc: core: add devm_mmc_alloc_host")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/20230217024333.4018279-1-yangyingliang@huawei.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>