pcmcia: at91_cf: move definitions locally

struct at91_cf_data is only used in the driver since all the platforms moved
to device tree, move its definition locally.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930184804.3127757-1-alexandre.belloni@bootlin.com