rge: fix multicast add/removal by using if_getdrvflags()

Using if_getflags() to check IFF_DRV_RUNNING is wrong;
if_getdrvflags() is required. This issue resulted in the
multicast filter not being updated.

This was an oversight by me in my initial port.

Thanks to danilo@ for reporting it and Oleg <oleglelchuk@gmail.com>
for the fix.

PR: kern/295176