rt2x00: correct address calc for queue private data

When calculating the offset to add to the queue entry base to get the
individual entry's private data area the base address must be treated
as a char * not a struct queue_entry so we can do byte oriented
pointer arithmetic with it.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>