mmc: at91_mci: remove the use of irq_to_gpio

Remove the use of irq_to_gpio() in the card detection interrupt
handler. The information is available in the board structure and
we can avoid using a function that has little meaning.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Chris Ball <cjb@laptop.org>