netdev: smsc911x: add missing __devexit_p() usage

The smsc911x_drv_remove() function is declared with __devexit, so the
assignment to the driver structure needs __devexit_p() wrappings to prevent
build failure when hotplug is disabled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>