cpmac: use resource_size()

The original code is off by one because we should start counting at
zero.  So the size of the resource is end - start + 1.  I switched it to
use resource_size() to do the calculation.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>