mtd: sh_flctl: Use cached register value for FLCMNCR

Instead of reading out the register, use a cached value. This will
make way for a proper runtime power management implementation.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>