brcmsmac: Don't weight AMPDU packets in txfifo

According to the comments this "reduces rate lag," but in reality the
only way this value is used is for determining whether or not any frames
remain to be transmitted. Therefore there's no reason for AMPDU packets
to receive any weighting.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>