mmc: host: Return an error when ->enable_sdio_irq() ops is missing

Even if the current WARN() notifies the user that something is severely
wrong, we can still end up in a PANIC() when trying to invoke the missing
->enable_sdio_irq() ops. Therefore, let's also return an error code and
prevent the host from being added.

While at it, move the code into a separate function to prepare for
subsequent changes and for further host caps validations.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220303165142.129745-1-ulf.hansson@linaro.org