ftmac100: use resource_size()

The calculation is off-by-one.  It should be "end - start + 1".  This
patch fixes it to use resource_size() instead.  Oddly, the code already
uses resource size correctly a couple lines earlier when it calls
request_mem_region() for this memory.

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