ARM: pxa: Add pxa320 PCMCIA check

On PXA320, there's only one PCMCIA slot available. Check for cases where the
user would want to register multiple. Also, rework failpath.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>