net: qualcomm: rmnet: Fix use after free while sending command ack

When sending an ack to a command packet, the skb is still referenced
after it is sent to the real device. Since the real device could
free the skb, the device pointer would be invalid.
Also, remove an unnecessary variable.

Fixes: ceed73a2 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>