rt2x00: Don't use unitialized rxdesc->size

rxdesc->size is unitialized before the desriptor has been read.
Move the truncation of the sk buffer to the moment all variables
have been initialized.

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>