rt2x00: Remove duplicate deinitialization

When rt2x00queue_alloc_rxskbs() fails rt2x00queue_unitialize()
will be called which will free all rxskb. So we don't need
to do this in the rt2x00queue_alloc_rxskb() function as well.

rt2x00queue_free_skb() unmaps the DMA but doesn't clear the
allocation flag. Since the code is copied from rt2x00queue_unmap_skb()
anyway (and that function does clear the flag) we might as well
use that function directly.

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