mtd: OneNAND: OMAP2/3: free GPMC CS on module removal

GPMC CS was not freed in omap2_onenand_remove() preventing the module
from reloading after removal.

Signed-off-by: Mika Korhonen <ext-mika.2.korhonen@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>