smsc911x: fix calculation of res_size for ioremap

fix size of remaped iomem, which is 1 byte to small
(e.g. mappes only 0xff bytes instead of 0x100)

Signed-off-by: Matthias Ludwig <mludwig@ultratronik.de>
Acked-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>