mmc: sdhci-acpi: remove unused struct sdhci_host variable

It was never used and introduce a warning

drivers/mmc/host/sdhci-acpi.c: In function 'sdhci_acpi_sdio_probe_slot':
drivers/mmc/host/sdhci-acpi.c:297:21: warning: variable 'host' set but
not used [-Wunused-but-set-variable]

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>