ibmvnic: Update TX pool cleaning routine

Update routine that cleans up any outstanding transmits that
have not received completions when the device needs to close.
Introduces a helper function that cleans one TX pool to make
code more readable.

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