mmc: sdhci-acpi: Fix IRQ 0

Zero is a valid IRQ number and is being used on some CHT tablets. Stop
treating it as an error.

Reported-by: Luke Ross <luke@lukeross.name>
Fixes: 1b7ba57e ("mmc: sdhci-acpi: Handle return value of platform_get_irq")
Cc: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>