mmc: pwrseq: use gpiod_get() instead of index 0

The gpiod_get() function expands to gpiod_get_index() with index 0
so it's better to use it since is easier to read and more concise.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>