b43legacy: fix suspend/resume

This patch makes suspend/resume work with the b43legacy driver.
We must not overwrite the MAC addresses in the init function, as this
would also overwrite the MAC on resume. With an all-zero MAC the device
firmware is not able to ACK any received packets anymore.
Fix this by moving the initializion stuff that must be done on init but
not on resume to the start function.
Also zero out filter_flags to make sure we don't have some flags
from a previous instance for a tiny timeframe until mac80211 reconfigures
them.

This patch by Michael Buesch has been ported to b43legacy.

Cc: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>