mmc: slot-gpio: switch to use flags when getting GPIO

When the slot GPIO driver gets the GPIO to be used for card
detect, it is now possible to specify a flag to have the line
set up as input. Get rid of the explicit setup call for input
and use the flag.

The extra argument works as there are transition varargs
macros in place in the <linux/gpio/consumer.h> header, in
the future we will make the flags argument compulsory.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>