mmc: davinci: remove incorrect NO_IRQ use

platform_get_irq() returns an error value on failure, not NO_IRQ,
so the error handling here could never work.

This changes the code to propagate the error value instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>