mmc: sdio: Fix potential NULL pointer error in mmc_sdio_init_card()

During some scenarios mmc_sdio_init_card() runs a retry path for the UHS-I
specific initialization, which leads to removal of the previously allocated
card. A new card is then re-allocated while retrying.

However, in one of the corresponding error paths we may end up to remove an
already removed card, which likely leads to a NULL pointer exception. So,
let's fix this.

Fixes: 5fc3d80e ("mmc: sdio: don't use rocr to check if the card could support UHS mode")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20200430091640.455-2-ulf.hansson@linaro.org