rt2x00: Data and desc pointer initialization

rt2500usb and rt73usb data and desc pointer initialization
was incorrect because it was using uninitialized variables
to determine the length.

In addition rt2500usb used skb_pull and removed the ieee80211
from each received frame instead of using skb_trim to remove
the device descriptor from the frame.

Finally this also fixes the descriptor override when 4 byte
aligning occured. We still need a completely valid descriptor
when using the TX/RX dumping capabilities in debugfs.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>