smsc911x: leave RX_STOP interrupt permanently enabled

smsc911x_set_multicast_list currently performs the only non-atomic
read-modify-write of INT_EN.  This patch permanently enables the
RXSTOP_INT interrupt, and changes the ISR to only conditionally run the
multicast filter workaround code.

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