avr32: clean up mci platform code

This patch does a few small cleanups around the atmel mci platform code
and in the atmel-mci driver.  The platform changes simply removes an
unused variable, uses the fact that by the end we always have some form
of platform data and notes that GPIO_PIN_NONE != 0.  This last point
could cause the incorrect attempt to twice reserve pin PA0.

While we've got the hood up, add linux/err.h to the atmel-mci.c include
list.  It needs it and generally pulls it by voodoo but I did once
stumble across a config which don't build.

This is against Linus' latest git.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>