mfd: Off by one calculating size for timberdale

I'm pretty sure that it should be + 1 here.  It's an off by one, because
we start counting at zero.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Richard Röjfors <richard.rojfors@pelagicore.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>