NET: sa11x0-ir: fix leak of tx skb

Ensure that we unmap and free a pending transmit skb when the interface
is stopped.  We rearrange the code a little bit to give all places a
similar layout when freeing the skb in both the completion and interface
stop paths - this gives some consistency to the code.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>