mmc: sdhci-spear: Handle return value of platform_get_irq

platform_get_irq() can fail here and we must check its return value.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>