mmc: pxamci: fix read-only gpio detection polarity

The commit converting pxamci to slot-gpio API inverted the logic of the
read-only gpio. Fix it by inverting the logic again.

Fixes: fd546ee6

 ("mmc: pxamci: fix card detect with slot-gpio API")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>