mmc: alcor: Fix a resource leak in the error path for ->probe()

If devm_request_threaded_irq() fails, the allocated struct mmc_host needs
to be freed via calling mmc_free_host(), so let's do that.

Fixes: c5413ad8

 ("mmc: add new Alcor Micro Cardreader SD/MMC driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/20200426202355.43055-1-christophe.jaillet@wanadoo.fr
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>