staging: brcm80211: remove assert to avoid panic since 2.6.37 kernel

The driver assumed it would receive skb packets from MAC80211 which
are not cloned. To guard this assumption an assert was placed in
the transmit routine. As of kernel 2.6.37 it turns out MAC80211 does
pass skb packets that are cloned. The assert is also not needed as
it does not lead to a failure state in our driver when the packet is
cloned. Therefore the assert can safely be removed.

> commit f8a0a781
> Author: Felix Fietkau <nbd@openwrt.org>
> Date:   Sat Dec 18 19:30:50 2010 +0100
>
>     mac80211: fix potentially redundant skb data copying
>
>     When an skb is shared, it needs to be duplicated, along with its data
>     If the skb does not have enough headroom, using skb_copy might cause t
>     buffer to be copied twice (once by skb_copy and once by pskb_expand_he
>     Fix this by using skb_clone initially and letting ieee80211_skb_resize
>     out the rest.
>
>     Signed-off-by: Felix Fietkau <nbd@openwrt.org>
>     Signed-off-by: John W. Linville <linville@tuxdriver.com>
>

Acked-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>