rt2500pci: restoring missing line

In kernel version 2.6.26-rc9 my wireless LAN card worked; but in the
released 2.6.26, my RaLink rt2500 card wouldn't associate.

Git-bisect led me to this patch:

	61486e0f68d1f8966c09b734566a187d42d65c54
	rt2x00: Remove ieee80211_tx_control argument from write_tx_desc()

I believe that there is a problem with that patch --- it
(inadvertantly) removes an extra line of code, that used to set the
DATABYTE_COUNT field.

This patch reinstates that line, and with it my card works again.

Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>