mmc: sdhci-pltfm: check return value of platform_get_irq()

The function platform_get_irq() can fail; it returns a negative error
code on failure.  A negative IRQ number will make sdhci_add_host() fail
to request IRQ anyway, but it makes sense to let it fail earlier here.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>