smsc911x: fix smsc911x_reg_read compiler warning

if this code path is ever hit, the platform_data struct isn't properly
configured with a bus width flag so the device won't work (hence the
BUG()).

This patch adds a dummy return statement to eliminate this compiler
warning:

drivers/net/smsc911x.c: In function 'smsc911x_reg_read':
drivers/net/smsc911x.c:148: warning: control reaches end of non-void function

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>