ibmvnic: Create init and release routines for the tx pool

Move the initialization and the release of the tx pool to their own routines,
and update them to do validation. This also adds validation to the release
of the long term buffer.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>