carl9170 firmware: modify the right beacon

Previously, the beacon address and length hardware registers
were (ab-)used as additional parameters for the beacon
modification functions. This idea worked fine, until I decided
to change the associated code and moved the beacon address and
length "set" _after_ the wlan_cab_modify_dtim_beacon...

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>