avr32: some mmc/sd cleanups

Minor cleanups for the MMC/SD support on avr32:

 - Make at32_add_device_mci() properly initialize "missing"
   platform data ... so boards like STK1002 won't try GPIO 0.

 - Switch over to gpio_is_valid() instead of testing for only
   one designated value.

 - Provide STK1002 platform data for the unlikely case that
   switches are set so first Ethernet controller isn't in use.
   (That's the only way to get card detect and writeprotect
   switch sensing on the STK1000.)

And get rid of one "unused variable" warning.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>