staging: brcm80211: bugfix for crash on heavy transmit traffic

With heavy transmit traffic, once in a while (range 15mins-1hr)
a tx packet was added to a full transmit queue. Under certain
conditions an other packet in the queue gets bumped to make room
for the new packet. This is not considered an error condition, but
normal operation. Despite that, there was an ASSERT(0) that caused
the driver to oops. The ASSERT(0) has been removed. Driver was tested
afterwards.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>