atmel-mci: Set MMC_CAP_NEEDS_POLL if no detect_pin

This allows the mmc core to detect card insertion/removal for slots that
don't have any CD pin wired up.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>