wireless mcast updates, ic->ic_update_mcast

[Sean Bruno]

My T61 has a wpi(4) card in it.  I noted that the console message "needs
muticast update" is emitted as it doesn't implement the required
function in its code and uses the default of the net80211 stack.  I
haven't noted any issues, but I took a journey through some of the
wireless drivers to see what the status of implementation was.  It seems
most of the drivers stub out the required function to get rid of the
warning.  

I perceive that with 6 exceptions, most of our wireless adapters don't
have an implementation, or worse, they implement a stub to squash the
warning.  Shouldn't these stub functions be removed?  Perhaps wrapp the
net80211 default behavior in bootverbose?

-- implemented ic_update_mcast() 
ath/if_ath.c -- implemented and working
usb/wlan/if_zyd.c -- implemented and working 
usb/wlan/if_wi.c -- implemented and working

-- sets a flag to default o/s behvavior?
mwl/if_mlw.c -- function coded out, most code if 0'd out
usb/wlan/if_upgt.c -- looks to default to whatever the o/s can do.
usr/wlan/if_run.c -- looks to default to whatever the o/s can do

-- stubbed out function to squash warning (bug?)
iwn/if_iwn.c -- stubbed out in if_iwn to remove default error
usb/wlan/if_rum.c -- stubbed out to remove default error
usb/wlan/if_urtwn.c -- stubbed out to remove default error
usb/wlan/if_urtw.c -- stubbed out to remove default error
usr/wlan/if_uath.c -- stubbed out to remove default error
usb/wlan/if_rsu.c -- stubbed out to remove default error
wtap/if_wtap.c -- stubbed out to remove default error

-- undefined, emits error correctly
wpi/if_wpi.c
bwi/if_bwi.c
bwn/if_bwn.c
ipw/if_ipw.c
malo/if_malo.c
ral/rt2560.c
ral/rt2860.c
ral/rt2661.c
wi/if_wi.c
usb/wlan/if_ural.c