mmc: slot-gpio: call gpiod_to_irq() only when MMC_CAP_NEEDS_POLL is unset

It is not efficient to call gpiod_to_irq() regardless the flag, then
ignore the returned irq if MMC_CAP_NEEDS_POLL.

Move gpiod_to_irq() after the MMC_CAP_NEEDS_POLL check.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>