staging: brcm80211: use fragment number provided in transmit frame

In the transmit path the field seq_ctrl is filled in, but the fragment
number was not properly determined.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>