staging: brcm80211: remove setting ssn to zero upon AMPDU_TX_START

The ssn should be given a valid value by the driver or it should not
override the value. Removed override to zero from the driver.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>