ibmvnic: Fix cleanup of SKB's on driver close

A race condition occurs when closing the driver. Free'ing of skb's
can race between the close routine and ibmvnic_tx_interrupt. To fix
this we move the claenup of tx pools during close to after the
sub-CRQ interrupts are disabled.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>